Barack Obama's hopes of re-election have been given a boost with projected wins in Pennsylvania, Wisconsin and New Hampshire.
The President is believed to have held Pennsylvania despite his Republican challenger Mitt Romney mounting a late charge to rally support there.
Wisconsin and New Hampshire, two of nine key swing states, are also being called for the Democrat, and in the exit poll in Iowa put him six points ahead.
However, other battlegrounds are still too close to call. Florida and Ohio are shaping up as the key counts, with Virginia, Colorado and North Carolina also in the balance.
The initial exit poll for Ohio, a state that has backed the winner in the last 12 elections, put the US President three points ahead.
No Republican has ever won the presidency without taking the bellwether state and a loss there could be a fatal blow to Mr Romney's campaign.
In Florida, where Republicans had been quietly confident, the lead constantly changed hands during the count but with 86% of votes checked, Mr Obama is still up by 1%.
:: READ MORE - Live Election Updates
Away from the main battleground states, expected victories racked up for each candidate.
Mr Obama took Illinois, Massachusetts, Maryland and Delaware and Mr Romney was the winner in Georgia, South Carolina, Tennessee and Indiana - a state won by the Democrat in 2008.
Both men are relying on their supporters to show up in high numbers and early indications are of a high turnout.
Pennsylvania officials said they expected about 70% of the state's nearly 8.5 million voters would have cast their ballot by the time polls close.
Despite the devastation caused by Hurricane Sandy and fears it could keep people from the polls, turnout was also high in New Jersey and New York.
In Hoboken, across the Hudson River from New York City, one makeshift polling station was 40 minutes late in opening, drawing complaints from people in line.
A poll worker told the crowd: "Please excuse the appearance of this place. Two days ago, it was under two feet (60cm) of water."
The current president sought to burn off his election-day nerves in traditional fashion, with a game of basketball with close friends.
Following the match, he returned to his house in Chicago and had dinner with his family.
Mr Romney spent the final hours making an eleventh-hour appeal for votes in Ohio and Pennyslvania before heading to Boston to see in the results.
Speaking on his campaign plane, he revealed he had already written his victory speech - which runs to 1,118 words - and that he had not prepared one in case he loses.
He told reporters he could lose because "nothing is certain in politics" but insisted he had "fought to the very end" to win the presidency.
"I feel like we have put it all on the field. We left nothing in the locker room. We have fought to the very end and I think that is why we will be successful," he said.
Even before election day began, more than 32 million early voters had made their choice. In a number of states, early voting levels were on track to exceed totals from 2008.
Mr Obama, waiting out the results in Chicago, has admitted: "We feel confident we've got the votes to win but it's going to depend ultimately on whether these votes turn out."
Tim Glisson, 57, leaving a polling station in the city, said: "Are you kidding? Obama," when asked who he had picked.
"I voted for him because of his character, his fairness - just doing the right thing," he said.
Fellow Obama supporter, Sandra Rendrich, 64, added: "I don't think any president can get done what he needed to get done in four years."
Elsewhere in the country, Romney supporters were equally confident of victory.
"We need to change this president," said Ruben Salazar, 72, a Cuban-American who woke up early to cast his ballot in Miami.
"I need a job for my wife, for my daughter, a better future for my grandsons. That's why I'll vote for Romney."
An unaffiliated group supporting Mr Romney's vice-presidential running mate Paul Ryan warned in a fundraising email that Democrats were voting in droves.
The message said: "We have to counteract this with a late surge of Republican and conservative Independent turnout."
The first ballots were cast just after midnight in the New Hampshire hamlet of Dixville Notch, where they were immediately counted.
In a sign of how close the race has become, the vote was tied for the first time ever with five for Mr Obama and five for Mr Romney.