Polls put Prime Minister David Cameron's Conservatives and Ed Miliband's Labour Party in a dead heat. Neither looks able to win a majority of Parliament's 650 seats.
Many voters are turning elsewhere — chiefly to the separatist Scottish National Party, which will dominate north of the border, and the anti-immigrant U.K. Independence Party. UKIP is third in opinion polls but Britain's electoral system means it can win at most a handful of seats.
If no party wins outright, it may take days or weeks of negotiation to forge a workable government.
The carefully stage-managed campaign lacked impromptu drama. But television debate appearances in which the public put questions directly to the politicians made plain that many distrust promises to safeguard the economy, protect the National Health Service from severe cutbacks and control the number of migrants from the European Union.
Cameron and Miliband were both up early to vote. The prime minister voted in his Oxfordshire constituency with his wife, Samantha, while Miliband cast his ballot alongside his wife, Justine, in northern England. Liberal Democrat leader Nick Clegg, the deputy prime minister in Cameron's government, walked hand-in-hand with his wife Miriam to the polls in the city of Sheffield.
Voters were making their voices heard all across this island nation of 64 million people. And signs of the unfolding political drama were abounded. The squares opposite Parliament were packed with temporary outdoor television studios, while commuters picked up newspapers urging voters to the polls.
In the bright early-morning sunshine in London, voters cast ballots at a polling station close to Parliament as police stood guard.
In one of London's poorest communities, Whitechapel, home to a large ethnic minority population, voters struggling in the wake of the worst recession since the 1930s wanted a change in leadership.
"The first priority is the economy, the second one is creating more jobs, and the third is living expenses — they're going higher and higher," said Shariq ul-Islam, 24, a student from Whitechapel's large Bangladeshi community.
Just a few minutes away is the City of London, the traditional financial district where many bankers earn salaries that their Whitechapel neighbours can only dream of.
Here, Christopher Gardner, a 34-year-old finance industry official, had trust in the Conservatives.
"There are some issues that have been caused by austerity previously — they're the only people that I'm confident will resolve that," he said.
In Scotland, voters turned out in droves, with lines at polling stations and a large turnout in the capital, Edinburgh.
"This is the most exciting election I can remember," said Lesley Milne, 48, from Glasgow. "It's time to shake up the politicians in London and the SNP are the people to do it."
About 50 million people were registered to vote, and early indications are that turnout will be high. There was an excitement about taking part — if only for the pleasure of being involved in a big national event.
Many used social media to spread the news that they've voted. Facebook said that for the first time in a British general election, users have access to the "I'm a Voter" button. More than 1.3 million people had used it as of Thursday morning.
Dogs and other creatures also featured on polling day. The hashtag #DogsAtPollingStations was one of the top 10 trends in Britain on Twitter, where many people posted fun photos of pups they took to — or spotted — at polling stops.
It's not just canines that offered a talking point on election day, when British broadcasters are banned from reporting on political news until polls close.
At the polling station where Miliband cast his vote, someone brought along a black lamb on a leash. Another voter linked an image of a horse, together with a woman in equestrian gear exiting the polling station.
Denmark's prime minister also popped into a British polling station — though she wasn't voting. Helle Thorning-Schmidt was in South Wales to support her husband, Stephen Kinnock, a Labour Party candidate for the constituency of Aberavon.
A video posted on YouTube showed the couple emerging from a polling station after Kinnock, the son of former Labour leader Neil Kinnock, cast his vote.
Asked how it felt to have a husband sitting in the parliament of another country, Thorning-Schmidt said she was happy for him because he worked hard campaigning.
"I don't know anyone who works as hard as Stephen," she said. "Today I am simply proud."
Asked how they would celebrate his win, she said: "Let's wait and see how things go today, but we will be very tense."
Sylvia Hui, Paul Kelbie and Martin Benedyk contributed to this story.Suggest a correction