Most of the UK's 650 constituencies are likely to declare seats in the small hours of Friday morning.
The final result may not be known until much later, with some two dozen constituencies not expected to be declared before noon.
Opinion polls during the campaign indicated that the opposition Conservatives had a slight advantage over the ruling Labour Party and the Liberal Democrats.
But with the election too close to call many analysts have suggested that there could be a hung parliament - in which no party wins an outright majority - for the first time since 1974.
Daniel Hamilton from Comres, a polling organisation, told Al Jazeera that the result was disappointing for the Liberal Democrats.
He said: "Well of course the Conservatives will be very disappointed.
"But I think the real losers here are the Liberal Democrats, they were widely thought to have fought an excellent campaign."
David Chater, Al Jazeera's correspondent at the Liberal Democrat's headquarters in London, also said there would be "great disappointment" amongst the party if the exit poll was correct.
He said: "They [the Liberal Democrats] were hoping to get up to 81 seats, which would definitely have put them in a major position to be kingmakers of any coalition, of any hung paliament, which does now look in prospect.
"So I think there will be a general discounting of this exit poll by the Liberal Democrats.
"They believe the night is going to be a long one, they believe they can win seats both from the Tories [Conservatives] and from the Labour party, and their hopes are still set very high."
Voter turnout appeared to be higher than usual, with some polling stations seeing queues before they opened.
Al Jazeera's Hamish Macdonald, reporting from a polling station in Ealing in West London, said the debates had "engaged an entirely new generation of voters".
"We've seen young people turning out in bars to watch the election debates and getting pretty excited over the whole thing," he said.
The election took place after an intense "marathon campaign" by party leaders, Alan Fisher, Al Jazeera's correspondent in London said.
"The campaign is all done, they can't kiss any more babies or shake any more hands. They've done all they can," he said.
More than 44 million people were registered to vote in the parliamentary election, with some 4,150 candidates standing for election across the country.
Voters in England also cast ballots on councillors for local authorities.
Thursday saw a number of dramatic events as voters headed to the polls, including an aircrash involving a candidate for the eurosceptic UK Independence Party, a minor party in the parliamentary poll.
Nigel Farage, former leader of the party, said he was "lucky to be alive" after escaping the incident, which occurred after the light plane became entangled in an election banner.
He suffered two broken ribs and a chip to his spine, the Press Association reported, while the pilot was believed to take more serious injuries.
|Polling stations reported higher
voter turnout than expected [AFP]
On Wednesday party leaders criss-crossed the country, the final day of campaigning, trying to win over undecided voters.
Gordon Brown, the incumbent prime minister, visited his native Scotland in a last-minute bid to win votes for his Labour Party, which has been in power for 13 successive years.
Cameron travelled from Scotland to a final rally in Bristol, southwest England, where he urged cheering crowds to cast their ballots and vote "for change".
Nick Clegg, the leader of the third-biggest party, the Liberal Democrats, made a last dash for votes across England in Eastbourne, Durham and Sheffield.
Clegg has been cast as a potential kingmaker in the event of a hung parliament after his performance in the country's first ever series of live televised debates between the leaders.
The number of Britons showing an interest in registering to vote surged after the debate, with the electoral commission reporting a significant increase in the number of people visiting its website and downloading voter registration forms.
Macdonald also said international election observers had criticised the fact that no identification is required for voters to cast their ballots.
"They say they go around the world, telling countries in the developing world that you need to turn up and vote. But in an election in the UK you don't, you just need a poll card."