LONDON ---suggested Prime Minister Theresa May's Conservatives would be the largest party, but predicted a hung Parliament with the Conservatives falling short of an overall majority.
As the country awaited results early into Friday morning, May spoke to supporters, saying Britain needed a "period of stability."
"Whatever the results are, the Conservative Party will ... ensure stability, so we can all as one country go forward together," May said.
Labour leader Jeremy Corbyn called on May to resign. Corbyn says the result means "politics has changed" and people have rejected Conservative austerity.
"The prime minister called the election because she wanted a mandate," Corybn said. "Well the mandate she's got is lost Conservative seats, lost votes, lost support and lost confidence. I would have thought that is enough for her to go actually."
More than three hours after polls closed in Britain's election, the first seat has changed hands, with Labour winning a constituency from the Scottish National Party.
Labour, the main opposition party, took Rutherglen and Hamilton West from the pro-independence SNP.
Of the other 20-plus seats that have declared, all stayed with the parties that held them before the election.
An exit poll suggests Labour is on course for a stronger-than-expected result, and Prime Minister Theresa May's Conservatives could lose their majority in Parliament.
That result would be a shock, overturning a big Conservative lead at the start of the campaign.
Labour Party leader Jeremy Corbyn said that "whatever the final result, we have already changed the face of British politics."
One of Britain's leading political figures says the exit poll projections following Britain's election, if accurate, would make it very difficult for a new government to be formed.
Former Treasury chief George Osborne, who now edits the Evening Standard newspaper, told ITV the exit poll would be "completely catastrophic" for the Conservative Party and May if it turns out to be accurate.
May called a snap election in hope of increasing the Conservatives' majority in Parliament and strengthening Britain's hand in EU exit talks.
Osborne said Thursday it would be hard for any party to put together a governing coalition if the poll is accurate.
The pound lost more than 2 cents against the dollar within seconds of the exit poll result, falling from $1.2955 to $1.2752 late Thursday. Some investors worry that the lack of a majority for the Conservatives, which are widely expected to top the poll, would weaken the next government's hand in the upcoming negotiations to leave the European Union.
Polls closed at 10 p.m. in an election for all 650 members of the House of Commons after May called an election three years ahead of schedule, at a time when her party was well ahead in the polls. But attacks that killed 30 people in Manchester and London have forced her to defend the government's record on terrorism, and this week she promised that if she wins she will crack down on extremism - even at the expense of human rights.
The northern English city of Newcastle claimed victory in the race to be the first to declare a result in Britain's general election.
An electoral officer announced that Labour had won the seat of Newcastle Central just before 11 p.m. Thursday, less than an hour after polls closed. That was six minutes ahead of the rival northern England seat of Houghton and Sunderland South, which declared first in 2015. That seat also went to Labour.
The two cities take the contest to declare first seriously, practicing rapid ballot-counting and rushing ballot boxes from polling stations to the count center.
Sunderland uses schoolchildren to run with the boxes, while Newcastle relies on sports students.
Rachel Sheard, who was casting her vote near the site of Saturday's van and knife rampage at London Bridge, said the election had not gone as expected -- and that it certainly wasn't about.
"I don't think that's in the hearts and minds of Londoners at the minute, (not) nearly as much as security is," said Sheard, 22. "It was very scary on Saturday."
Eight people were killed near London Bridge when three men drove a van into pedestrians then stabbed revelers in an area filled with bars and restaurants. Two weeks earlier, a suicide bomber killed 22 people as they were leaving a concert in Manchester, and five people died during a vehicle and knife attack near Parliament on March 22.
The attacks have left Britain on high alert. The official threat level from terrorism stands at "severe," the second-highest rating, indicating an attack is "highly likely."
When May called the election seven weeks ago, she was seeking to capitalize on opinion polls showing that her Conservatives had a wide lead over the opposition Labour Party. She became prime minister through a Conservative Party leadership contest when her predecessor, David Cameron, resigned after voters backed leaving the EU. The time seemed right to seek her own mandate from the British people. She argued that increasing the Conservative majority in Parliament would strengthen Britain's hand in EU exit talks.
But things did not go to plan.
Brexit failed to emerge as a major issue in the campaign, as both the Conservatives and Labour said they would respect voters' wishes and go through with the divorce.
May, who went into the election with a reputation for quiet competence, was criticized for a lackluster campaigning style and for a plan to force elderly people to pay more for their care, a proposal her opponents dubbed the "dementia tax." As the polls suggested a tightening race, pollsters spoke less often of a landslide and raised the possibility that May's majority would be eroded.
In her final message to voters, May tried to put the focus back on Brexit.
"I can only build that better country and get the right deal in Brussels with the support of the British people," she said. "So whoever you have voted for in the past, if that is the future you want then vote Conservative today and we can all go forward together."
Corbyn, an old school left-winger widely written off at the start of the campaign, has drawn thousands of people to upbeat rallies and energized young voters with his plans to boost public spending after years of Conservative austerity.
His calls for increased spending on the National Health Service, schools and police, as well as the nationalization of railroads and utilities, have proven popular -- but it would take a huge swing in voter support for Labour to unseat the Conservative government.
Corbyn told supporters at his final rally that Labour's campaign had "changed the debate and given people hope. Hope that it doesn't have to be like this; that inequality can be tackled; that austerity can be ended; that you can stand up to the elites and the cynics. This is the new center ground."
While the gap between the two parties has narrowed, virtually all polls suggest the Conservatives will retain control of Parliament. A high turnout is seen as Labour's best hope of eroding the Conservative majority.
The Conservatives held 330 seats in the last Parliament, compared with 220 for Labour, 54 for the Scottish National Party and nine for the Liberal Democrats.
Security has dominated the late stages of the campaign, after the attacks in Manchester and London. May said this week that she would consider rewriting human rights legislation if it gets in the way of tackling extremism.
Corbyn, meanwhile, accused Conservatives of undermining Britain's security by cutting the number of police on the streets.
While security was on many voters' minds, it was far from the only issue.
"It's important, but it's only one issue amongst several," said 68-year-old Mike Peacroft. "I wouldn't necessarily say it's at the top. Obviously at my end of the (age) spectrum I'm more interested in things like pensions and so forth, NHS health care -- plus schooling, those are really my main concerns."
It rained intermittently across much of the country on polling day, but experts said it would likely not affect turnout.
"We live in a country where a bit of drizzle is commonplace," said John Curtice, an election expert at Strathclyde University.