President Barack Obama has been re-elected to a second term, defeating Republican challenger Mitt Romney.
With results in from most states, America's first black president has secured the 270 votes in the electoral college needed to win the race.
Mr Obama prevailed despite lingering dissatisfaction with the economy and a well-funded challenge by Mr Romney.
Many states are still reporting results, and Mr Obama's margin of victory was not yet certain.
With swing states Virginia, Florida and Colorado still too close to call, Mr Obama has won 281 electoral votes to Mr Romney's 203.
Under the US constitution, each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-take-all state contests - becomes president.
The popular vote, which is symbolically and politically important but not decisive in the race, remains too close to call.
On Tuesday, the president held the White House by assembling solid Democratic states and a number of important swing states such as Iowa, Pennsylvania, Michigan, Minnesota and Wisconsin. His narrow victory in Ohio, a critical Mid-Western swing state, sealed the victory.
Mr Romney won North Carolina and Indiana, two states Mr Obama won in 2008, as well as the solid Republican states.
But he was unable to win in Ohio or other states needed to breach the 270 threshold.
The polls are closing in the US presidential election, with results streaming in from across the country.
President Barack Obama has won swing states Michigan and Pennsylvania, as well as many others he won in 2008.
Republican Mitt Romney has carried several solid Republican states, also taking Indiana from Mr Obama.
Tens of millions of voters cast ballots on Tuesday. The winner could be declared within hours, especially as other swing states report results.
Pre-election polling suggested a neck-and-neck race but gave the president a slender lead in some swing states.
Polling stations began closing in eastern states at 18:00 EST (23:00 GMT), with the crucial states of Virginia and Ohio closing at 19:00 EST and 19:30 EST respectively.
In those states and elsewhere, election workers at polling stations with long queues allowed voting to continue past the official closing.
In one of the first swing states to fall, Mr Obama won Michigan, repeating his 2008 victory there.
Mr Romney's father George was governor of Michigan in the 1960s and Mitt Romney was born and raised there.
But the president's 2009 rescue of the car industry helped seal the deal with the state's voters, analysts say.
Mr Obama also took Pennsylvania, continuing a Democratic streak dating back to 1992. He also held New Hampshire and Minnesota, states he won in 2008 but which the Republicans hoped to take from him.
Elsewhere, Mr Romney won Alabama, Arkansas, Georgia, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Montana, Nebraska, North Dakota, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, Utah, West Virginia and Wyoming.
Mr Obama has taken Connecticut, Illinois, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Pennsylvania, Rhode Island, Vermont, and Washington DC.
Of the states called so far, Indiana switched hands from 2008, when Mr Obama narrowly won it. Mr Obama also failed to hold a single Nebraska electoral vote he won in 2008.
Obama greeted supporters outside a campaign office in Chicago on Tuesday
After a hard-fought race that began nearly two years ago and cost more than $2bn (£1.3bn), national polls by Washington Post/ABC News and the Pew Research Center both give Mr Obama a three-point edge over his rival.
In areas of New Jersey and New York that were damaged by storm Sandy a week ago, turnout was described as heavy. One high school being used as a shelter for displaced families doubled up as a voting centre.
Across the US, voting took place against a variety of backdrops - from a launderette and an internet cafe in Chicago to a lifeguard station in Los Angeles and car dealership in Columbus, Ohio.
Obama 'very good'
Mr Obama was the first of the two rivals to cast their ballot by voting last month in his adopted home city of Chicago, and becoming the first sitting president to vote before election day.
Mr Romney and his wife Ann voted in the Boston suburb of Belmont, Massachusetts, shortly before 09:00 local time on Tuesday. He told reporters he felt "very good".
Each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-take-all state contests - becomes president.
Also on Tuesday's ballot are 11 state governorships, a third of the seats in the 100-member US Senate and all 435 seats in the House of Representatives.
Republicans are expected to keep control of the House, while Democrats are tipped to do the same in the Senate.
Legal battles feared
With voter turnout seen as vital for both candidates, campaigning continued on Tuesday, but at a slower pace.
Both men gave radio interviews, and Mr Romney hit the campaign trail again, with visits to Ohio and then Pittsburgh, Pennsylvania.
Romney visited a campaign call center in Green Tree, Philadelphia
Analysts tried to determine whether Mr Romney's last-minute campaigning showed strength or desperation, while the candidate reportedly told aides he wanted to be active on election day instead of just sitting around waiting for results.
The Republican was due to return to Boston later for an election night rally.
Mr Obama, who will hold an election night rally at a convention centre in Chicago, visited a campaign office in the city to talk to volunteers. He praised his Republican rival's "spirited campaign".
With observers anticipating a close race, both sides have readied teams of lawyers for possible legal fights, especially in the critical battleground state of Ohio.
Some analysts fear the election will not be decided on Tuesday night if the state's vote becomes mired in legal battles.
The BBC is providing full online live results of the US presidential election. More details here .
There's a real party atmosphere here now. Inspirational videos, campaign surrogates cheerfully doing the rounds. #election2012 @BBCNewsUS@davieseliz1 min ago
Obama now at 172, Romney at 163 - need 270 to win. The 'big' swing states of Florida (29 votes) Ohio (18 votes) still unknown.@JoanSoley1 min ago
#Election2012 Romney's path to victory narrowing - needs Florida's 29 votes@LauraTrevelyan4 mins ago
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Tens of millions of Americans head to the polls on Tuesday to decide whether to re-elect President Barack Obama or hand the job to Republican Mitt Romney.
The voting ends a hard-fought race that began nearly two years ago and has cost more than $2bn (£1.3bn).
Polls will begin closing in eastern states at 19:00 EST (00:00 GMT) - a winner could be known by midnight.
Polls show the race is neck and neck, although the president holds a slender polling lead in crucial swing states.
National polls by Washington Post/ABC News and the Pew Research Centre both give Mr Obama a three-point edge over his rival.
As many as 30 million voters have already cast their ballots, with more than 30 states allowing either absentee voting or in-person early voting.
On the stroke of midnight, the first votes were cast and quickly counted in the tiny village of Dixville Notch in New Hampshire. They resulted in a tie with five votes each for Mr Obama and Mr Romney.
'Work not done'
Mr Obama has already voted in his adopted hometown of Chicago, becoming the first sitting presidential nominee ever to vote early. Mr Romney is expected to cast his own ballot in Belmont, Massachusetts, later on Tuesday.
The election is decided by the electoral college. Each state is given a number of electoral votes in rough proportion to its population. The candidate who wins 270 electoral votes - by prevailing in the mostly winner-take-all state contests - becomes president.
Also on Tuesday's ballot are a handful of state governors, one third of the seats in the 100-member US Senate and all 435 seats in the House of Representatives.
Republicans are expected to keep control of the House, while Democrats were tipped to do the same in the Senate.
The presidential candidates spent Monday frantically criss-crossing the crucial battleground states including Ohio, Florida, Iowa and Virginia, making final appeals to voters. Their task: Push their own supporters to the polls while persuading the sliver of undecided voters to back them.
In speeches, Mr Romney kept up his attack on Mr Obama's record, reciting a litany of statistics he says illustrate the president has failed to lift the US economy out of the worst downturn since the Great Depression that followed the stock market crash of 1929.
"If you believe we can do better, if you believe America should be on a better course, if you're tired of being tired... then I ask you to vote for real change," Mr Romney told a rally in a Virginia suburb of the capital, Washington DC.
The president appeared at rallies with singer Bruce Springsteen and rapper Jay-Z. He acknowledged frustration with the still-lagging economy but told voters "our work is not done yet".
"We've come too far to turn back now," the president said in Ohio. "We've come too far to let our hearts grow faint... We'll finish what we started. We'll renew those ties that bind us together and reaffirm the spirit that makes the United States of America the greatest nation on Earth."
Legal battles feared
With observers anticipating a close race, both sides have readied teams of lawyers for possible legal fights, especially in the critical battleground state of Ohio.
Some analysts fear the election will not be decided on Tuesday night if the state's vote becomes mired in legal battles.
On Tuesday Mr Romney is to hit the campaign trail again with events in Pittsburgh, Pennsylvania and Cleveland, Ohio, before holding an election night rally in Boston.
Mr Obama will hold his own election night rally at a convention centre in Chicago.
The BBC will be providing full online live results of the US presidential election on 6 November. More details here
Are you a voter in one of the swing states? Send us your comments on the election campaign using the form below.
US presidential rivals Barack Obama and Mitt Romney are approaching the final day of their election battle in a frantic fight for swing state votes.
The pair spent Sunday addressing crowds across the country, with Mr Romney speaking in Pennsylvania, a state his aides insist he can now win on Tuesday.
Mr Obama held rallies in New Hampshire and Florida and carried on to Ohio and Colorado in the evening.
Analysts say the election will come down to a handful of swing states.
Mr Obama and Mr Romney are running almost neck-and-neck in national polls, but polls of many key battlegrounds show Mr Obama narrowly ahead.
However, neither camp is exuding absolute confidence, the BBC's North America editor Mark Mardell reports.
The campaign has been most intense in Ohio, which no Republican has ever lost and still made it to the White House.
Race 'tightening'
A final poll published on Sunday by Ohio's Columbus Dispatch gave Mr Obama a 2% lead - 50% to 48% - over his rival, within the margin of error.
Both candidates visited the Buckeye State on Sunday, with Mr Romney telling crowds in Cleveland that Mr Obama has failed in his pledge to be a "post-partisan" president and criticising his record.
"He's been divisive, blaming, attacking, dividing and - by the way - it's not only Republicans that he refused to listen to, he also refused to listen to independent voices."
Later on Sunday he spoke in Morrisville, Pennsylvania, his first visit there in the final stages of the campaign. Democrats say the Romney team's last-minute decision to campaign in the state is a sign of desperation, but polls do show a tightening race.
"The people of America understand we're taking back the White House because we're going to win Pennsylvania," Mr Romney told the crowd in Morrisville.
Mr Obama made another visit to Ohio on Saturday, a state where he holds a slim lead in opinion polls
Mr Obama made another appearance in Cincinnati, Ohio where Stevie Wonder opened a huge evening rally.
Earlier in the day at a rally in Concord, New Hampshire, Mr Obama said: "We have come too far to turn back now."
He said he would work across party lines to break the political gridlock in Washington, but would not compromise on priorities such as healthcare and college financial aid.
On Monday morning Mr Obama is scheduled to appear in Madison, Wisconsin, accompanied by Bruce Springsteen, before going on to Iowa and Ohio.
Mr Romney is due in Florida - where polls suggest he is ahead- in Virginia, New Hampshire and Ohio.
Activists have been stepping up efforts across the crucial swing states.
In Wisconsin, student volunteers have been putting in 14-hour days in an effort to deliver the state for Mr Obama, the BBC's Paul Adams reports from Madison.
An opinion poll on Sunday for ABC News and the Washington Post put the two candidates at 48%, with even voters who term themselves independents split evenly on 46%.
Mr Romney remains favoured among whites, seniors and evangelical Christians; Mr Obama among women, non-whites and young adults.
The president also remains slightly ahead in most of the nine-or-so swing states that will determine the election.
Opinion polls published on Saturday showed him well-placed in Iowa, Nevada and Ohio, but most remain within the polls' own margins of error.
The election is run using an electoral college. Each state is given a number of votes based on its population. The candidate who wins 270 electoral college votes becomes president.
The BBC will be providing full online live results of the US presidential election on 6 November. More details here
Are you a voter in one of the swing states? Send us your comments on the election campaign using the form below.
Obama supporters cheered the results 
