Polls have closed in the key New Hampshire presidential primary, amid expectations of a record turnout.
John McCain has won the Republican presidential primary, according to projections from US media outlets.
The state secretary, Bill Gardner, has estimated that at least half a million people may have cast ballots.
The Democratic race is neck-and-neck between Barack Obama and Hillary Clinton, according to exit polls.
The first votes counted, and opinion polls, put Barack Obama ahead in the Democratic race and John McCain ahead for the Republicans.
The state secretary, Bill Gardner, has estimated that the turnout in the vote could break records, with some half a million people having cast ballots.
Candidates are aiming to build momentum before more than 20 states hold polls on 5 February, known as Super Tuesday.
Candidates are aiming to build momentum before more than 20 states hold polls on 5 February, known as Super Tuesday.
Turnout up
Analysts say New Hampshire's large bloc of independents - about 45% of registered voters - could be key to swinging the primary, the second in a series of state-level votes and caucuses through which the parties choose their candidates for the US presidency.
Analysts say New Hampshire's large bloc of independents - about 45% of registered voters - could be key to swinging the primary, the second in a series of state-level votes and caucuses through which the parties choose their candidates for the US presidency.
Today you can make your voice heard - you can insist that change will come Barack Obama
Mr Obama, in particular, showed strong appeal at last week's Iowa caucuses among such voters, who are registered as neither Republican nor Democratic.
Mr Obama, in particular, showed strong appeal at last week's Iowa caucuses among such voters, who are registered as neither Republican nor Democratic.
The BBC's Justin Webb in New Hampshire says Mr Obama's team are now expecting a "crushing victory" in the state.
While Iowa and New Hampshire - two of the earliest contests - do not necessarily produce a winner, they can boost contenders' chances of winning ahead of primaries in larger states.
'Absolutely huge'
Two tiny hamlets, Dixville Notch and Hart's Location, traditionally the first to vote in New Hampshire, opened their polling stations at midnight giving small, early victories to Mr Obama and Mr McCain.
Two tiny hamlets, Dixville Notch and Hart's Location, traditionally the first to vote in New Hampshire, opened their polling stations at midnight giving small, early victories to Mr Obama and Mr McCain.
Most of the rest of the state began voting at 0600 local time (1100 GMT). Polls closed at 2000 (0100GMT Wednesday) and first results are expected shortly.
Most of the rest of the state began voting at 0600 local time (1100 GMT). Polls closed at 2000 (0100GMT Wednesday).
New Hampshire voters explain who will get their vote in the race for the White House In pictures
Turnout has exceeded expectations, helped by unusually mild weather for this time of year in New Hampshire.
Turnout has exceeded expectations, helped by unusually mild weather for this time of year in New Hampshire.
Candidates from both parties have spoken at a number of campaign events throughout the state.
At a rally at Dartmouth College, Mr Obama told cheering supporters: "Today you can make your voice heard - you can insist that change will come.
"The American people have decided for the first time in a very long time to cast aside cynicism, to cast aside fear, to cast aside doubts."
Democrat Senator Hillary Clinton and the Republicans Rudy Giuliani and Mike Huckabee were among the candidates who turned up at polling stations throughout the day in last-minute bids for votes.
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After finishing third in Iowa, the former first lady Mrs Clinton trailed Mr Obama by as many as 13 points in one state-wide opinion poll.
Joined by her daughter, Chelsea, and husband, former President Bill Clinton, Mrs Clinton said she was feeling "really good" and was "going to work all day to get the vote out".
Excited crowds waited to greet her at a polling station at a school in Concord. Among them were dozens of children, with whom she posed for pictures, as well as local voters.
Mrs Clinton has seen her lead in the polls eclipsed by Mr Obama but has vowed to "go on".
'Starting below the bottom'
Mr Huckabee, who is currently polling third in the Republican contest, was in a jocular mood, even asking Rudy Giuliani for his vote.
"We started below the bottom. For us to come in the top four would be a win for us," said the former Arkansas governor and Baptist minister, who was a virtual unknown until recently.
John McCain has resumed his lead in the pollsArriving at the site moments later, Republican contender Mitt Romney said: "The Republicans will vote for me. The independents will get behind me."
After a disappointing second-place showing behind Mr Huckabee in Iowa, Mr Romney needs a strong showing in New Hampshire to keep his campaign alive.
Both are threatened by a resurgent Mr McCain who, after seeing his campaign nearly derail last summer, has concentrated much of his time and money on New Hampshire.
Other Republican candidates have been looking beyond the New Hampshire primary to focus on states that go to the polls later.
Former New York mayor Mr Giuliani has been focusing on Florida's 29 January contest, hoping to gain momentum going into Super Tuesday.
Actor and former Senator Fred Thompson has been campaigning in South Carolina, where Republicans vote on 19 January.
