Britain is heading for a hung parliament with David Cameron's Conservatives the largest party, according to an exit poll.
Tory leader David Cameron was left tantalisingly close to the door of Downing Street with his party on 307 seats - 19 short of an overall majority.
Gordon Brown's Labour would have 255 seats and Nick Clegg's Liberal Democrats would have 59 seats, according to the poll.
The BBC/ITV News/Sky News poll interviewed 18,000 voters, who had already cast their ballot, at 130 polling stations across the UK.
If the poll was mirrored in tonight's actual results, it would mean the Liberal Democrats would have made no progress since the 2005 election despite Mr Clegg's strong campaign.
Today's closely-fought poll was inevitably seen as a verdict on Labour's 13 years in power, and the premier's three years in office.
It was billed as the most nail-biting election since 1992, when John Major defied predictions of a hung parliament to secure a Conservative majority.
And it followed an apparent surge of support for the Lib Dems after the first of the three TV election debates between the candidates for No 10, which featured for the first time in a British election.
That confounded the expectations of a smooth path to victory for Mr Cameron's Tories.
But if tonight's exit poll is right, the Lib Dem bubble has burst and the party was unable to translate opinion poll ratings into Westminster seats.
The three party leaders were in predictably buoyant mood as they cast their votes earlier today, although Tory leader David Cameron's arrival at his local polling station was delayed by more than two hours after pranksters scaled its roof.
Former Ukip leader Nigel Farage, standing against Commons Speaker John Bercow in Buckingham, suffered broken ribs and a chipped spine when the light aircraft he was campaigning in crashed to the ground.
The MEP was pulled bloodied and dazed from the crushed cockpit of the plane which was towing a banner bearing the slogan "Vote for your country - Vote Ukip".
Mr Farage is being treated at John Radcliffe Hospital in Oxford, where he is likely to remain for a few days, ruling out an appearance at his count.
More than 44 million people were registered to vote for the 650 MPs of the new Parliament, with polling stations open from 7am until 10pm. Voters were also electing councillors in 166 local authorities across England.
Responding to the exit poll, Labour's deputy leader Harriet Harman told BBC News: "It's obviously going to be very close.
"What is clear is that the country is going to need a strong and stable government to take us through the recession.
"And the country hasn't turned overwhelmingly to the Conservatives and given them the trust and confidence ... predicted a year or so ago."
Liberal Democrat deputy leader Vince Cable described the outcome of the exit poll as "very strange" and insisted they had been "horribly wrong" in the past.
Conservative Party chairman Eric Pickles told ITV: "I think we're going to see a very interesting night.
"If I was a member of the public, I would be staying up all night because I think we are going to see things that occur above what you are saying and things that are going to occur below.
"And I think we are going to see an enormous amount of churning between the political parties."
Put to him that the Tories would have to do deals, he responded: "We have looked at the figures but I'm an old-fashioned sort of chap, and I want to see what the public have said."
Former Liberal Democrat leader Lord Ashdown of Norton-Sub-Hamdon Paddy Ashdown told ITV that the exit poll predictions would be "inaccurate" because they did not take into account postal votes.
And former Labour leader Lord Kinnock said it was "time to express hopes".
"Looking at those figures, I think we can say with certainty that the figure for the Liberal Democrats is wrong," he said.
"My guess from this point is the Conservative figure will be slightly lower and ours will be slightly higher."
Shadow schools secretary Michael Gove pointed out to BBC News the polls had only just closed.
"We need to see the real results," he said.
"Exit polls can show rogue results.
"This is another piece of evidence of a comprehensive rejection of Gordon Brown and the Labour Government and a strong vote for change."
The country needed a "strong, responsible and stable" government to see it through the times ahead, he said.
Mr Gove said he "absolutely believed" it was possible for the Tories to win a majority, adding there may be some "quirky" results in parts of the country.
Home Secretary Alan Johnson admitted that the Tories "seemed to have done very well".
But he stressed that the parties would have to look at the "will of the British people" before forming any coalition.
Mr Johnson told Sky News: "We're at the scale of a hung Parliament territory where the Tories seem to have done very well.
"We'll need to see. It sounds like our vote's held up and the Liberal Democrats - the air was coming out of the tyre, it's gradually seeped out from that first debate."
He added: "I've got no problem with deals and coalitions and all the rest of it. But it has to be in the right circumstances.
"We have to respect the will of the British people, and that all depends on how that finally pans out and what the percentage share of the vote is, although seats are important."
Mr Johnson added that he would be "surprised" if turnout was not up by at least 5%, and said it could have risen by as much as 10%.