Gordon Brown held his seat of Kirkcaldy and Cowdenbeath, using his acceptance speech to promise an investigation into the polling chaos.
The Electoral Commission has said an investigation will take place into reports that people have been turned away from polling stations.
With voter turnout higher than expected, the Commission says the guidelines are clear, but accepts the system is flawed. There is speculation that this could lead to legal challenges where reults are close-run.
Latest reports suggest some in Hackney South were turned away an hour before polls closed, with others missing out due to long queues.
Exit polls suggest Tory leader David Cameron would fall 19 seats short of an overall majority, leaving the country with a hung parliament.
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The Liberal Democrats, Britain's traditional third party, could emerge as kingmakers in a power-sharing deal after a surge during a grueling month-long campaign..
Early results could start coming in as soon as an hour later, but the bulk are expected to be declared from around 3:00 am.
The final result may not be known until much later if all hangs on a handful of seats, as some two dozen constituencies are not expected to be declared before noon on Friday.
Election day was marked by a plane crash which injured a high-profile anti-Europe candidate and a protest outside the polling station where Cameron voted.
Several eve-of-election polls suggested the Conservatives had a clear lead over Brown's ruling Labour Party and Nick Clegg's Liberal Democrats.
But they indicated that under Britain's first-past-the-post system, Cameron's Tories could fall short of an overall majority in the House of Commons, setting up the first hung parliament since 1974.
A poll by ICM for the Guardian newspaper predicted Conservative support had increased slightly to around 36 percent, with Labour unchanged on 28 percent, while the Lib Dems had fallen back to 26 percent.
That would roughly equate to 283 seats for the Tories, 253 for Labour and 81 for the Lib Dems. A total of 326 seats are needed for an overall majority.
Such an outcome would spark a scramble for power, with Cameron seeking a partner to govern or doing so through a minority government, possibly with the support of a handful of lawmakers from Northern Ireland.
More than 44 million voters were eligible to cast ballots, with observers predicting turnout could be as high as 70 percent after an unusual campaign transformed by the first televised leaders' debates in a British election.
A smiling Cameron and his wife Samantha voted in the picturesque village of Spelsbury in his Witney constituency in Oxfordshire, west of London.
Earlier, two pranksters climbed on to the roof of the polling station and unfurled a banner drawing attention to Cameron's education at the elite fee-paying Eton College, which has produced 18 British prime ministers.
Brown, who has been fighting for his political life in a frantic week of campaigning, was accompanied by his wife Sarah as he voted in steady drizzle in his constituency in Fife, north of Edinburgh.
Clegg, whose surprise strong performance in the first TV leaders' debate gave his Liberal Democrats a massive boost, cast his ballot in Sheffield, northern England, where he was elected for the first time in 2005.
Nigel Farage, a high-profile candidate for the eurosceptic UK Independence Party (UKIP), suffered minor head injuries and needed heart tests after the light aircraft he was travelling in crashed at an airfield in Northamptonshire.
His campaign manager said the pilot desperately tried to call for help in the seconds before impact.
"Apparently the plane nose-dived. We had a banner attached to the back of the plane which basically got wrapped around the tail," said Chris Adams. "It's all a bit of a shock, especially on polling day."
Brown, who took over from Tony Blair in 2007, ended his campaign in his native Scotland, issuing a last-ditch plea to wavering voters to back Labour as the best party to safeguard the country's fragile recovery from deep recession.
"At this moment of risk to our economy, at this moment of decision for our country, I ask you to come home to Labour," he said.
Clegg pleaded with voters to back him and seize a "once in a generation opportunity to do things differently".
Late Thursday, one Labour candidate who embarrassed Brown on the eve of the election by branding him Britain's "worst prime minister" said he would not bother to go the election count later Thursday night.
"I had no chance of winning anyway, but I've been disowned by the party... I've decided not to go to the count -- I'll stay at home and watch the BBC," he said.
