Al Jazeera's Alan Fisher, reporting from London's Downing Street, said the party leaders were voting in their home constituencies after an intense "marathon campaign".
"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.
"We're told there's still a huge number of undecided voters, somewhere in the region of two million people. So the election will go to the very last vote I'm sure.
"We are hearing from constituencies across the country that turnout is higher than expected."
More than 44 million people are registered to vote in the parliamentary election.
Voters in England will also elect councillors for local authorities.
Almost 4,150 candidates are standing for election across the country.
A light aircraft carrying a candidate for the euroskeptic UK Independence Party, a minor party in the parliamentary poll, crashed during an election-day stunt.
Officials said Nigel Farage was treated in hospital after the accident.
"He is OK so it's not life-threatening. The show goes on,'' Chris Adams, his aide, told
The plane was hired to tow a party banner over the constituency northwest of London where Farage is running for a seat in the House of Commons.
|Polling stations were reporting higher
voter turnout than expected [AFP]
Party leaders criss-crossed the country on Wednesday, 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.
Al Jazeera's Hamish Macdonald, reporting from a polling station in Ealing in West London, said the debates had "engaged an entirely new generations of voters".
"We've seen young people turning out in bars to watch the election debates and getting pretty exited over the whole thing," he said.
Poll officials in Ealing said their polling stations had never been so busy as this year, with some people were queueing outside before 7am.
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."