-- Final polls suggest May on course to increase majority

-- Security tight five days after London Bridge attack

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-- May seeks stronger mandate ahead of Brexit talks

-- Poll lead over Labour has shrunk during campaign

Britons streamed to the polls for the third time in two years on Thursday after a campaign overshadowed by terror attacks and the uncertainty of Brexit.

Prime Minister Theresa May called the vote in April, when opinion poll ratings for her and her centre-right Conservative party were sky high, presenting herself as the strong leader to steer the country through tough negotiations to leave the EU.

Although she is strong favourite to keep her job, Islamist attacks in London and Manchester have put her under pressure over her six years as interior minister, while campaign missteps have dented her reputation as a safe pair of hands.

Labour opponent Jeremy Corbyn, an anti-war campaigner deemed unelectable by a majority of his own lawmakers, has run an energetic campaign, promising change and an end to austerity.

Security was high as millions of voters cast their ballots in polling stations ranging from schools and public buildings to churches, pubs and even a windmill and a launderette.

An exit poll at 2100 GMT will give an indication of the outcome, although final results will not emerge until early Friday.

- Pollsters wary -

Polling experts -- many of whom failed to predict the historic referendum vote to leave the European Union last year -- are now wary of calling the outcome.

But predictions of her expected margin of victory vary widely, and one shock forecast model even predicted May could lose her majority of 17 in the 650-seat House of Commons.

While May has been touring target seats around the country, delivering slogan-heavy speeches to small groups of hand-picked activists, Corbyn has drawn large crowds to open-air rallies.

The 60-year-old vicar's daughter has presented herself as uniquely qualified for the Brexit talks and said her 68-year-old rival would be "alone and naked in the negotiating chamber".

"Get those negotiations wrong and the consequences will be dire," she said on a final campaign stop on Wednesday.

Corbyn, a veteran socialist who has never held ministerial office and defied the odds to win the Labour leadership two years ago, urged supporters in Glasgow to think big on his last day of campaigning.

"Wouldn't it be great if on Friday we woke up to... a Labour government that will be a government for all of our communities across the whole of the country," he said.

- 'Little confidence' -

It is the third time Britons have been called to vote since 2015, twice for a general election and once for the EU referendum, and voter fatigue appeared to be an issue among some.

"I have little confidence in anybody," said voter Simon Bolton, 41, in east London. "I think we lack quality in terms of who we can choose, it is very limited."

The election is May's first since taking office after Britons voted by 52 percent to leave the European Union after four decades of membership.

"I want another five years of stability, and a stronger hand in Brexit negotiations," said Dave, 29, in the London neighbourhood of Hackney.

"The main issue for me is getting along with Brexit," added Fabrizio, 42, in the wealthy London district of South Kensington.

But the Conservative government's record on cutting funding for health and education have also featured strongly in the campaign, to the benefit of Labour.

The Conservatives were also damaged by a manifesto plan for elderly care that would see some pay more.

"The cuts that have been happening, the austerity on education and the National Health Service, it weights on my vote," said Saskia Green 39, a voter in north London.

The campaign was hit by a suicide bombing at a Manchester concert on May 22, which killed 22 people including seven children, followed by Saturday's knife and van attack in central London, which left eight dead.

Campaigning was twice suspended in the aftermath of the attacks, which May blamed on "evil" Islamist ideology.

- Uncomfortable questions -

The Conservatives are traditionally strong on defence and security, and have sought to exploit Corbyn's anti-nuclear stance and his alleged past support for Irish paramilitaries.

But May also faced uncomfortable questions over cuts to police numbers during her time as interior minister.

Security on election day was reviewed following the London attack, with the city's Metropolitan Police force implementing a "specialist and highly flexible operation" which it said could be deployed as needed.

May and Corbyn both cast their votes in their respective constituencies of Maidenhead, southern England, and Islington, north London.

In a frequently ill-tempered campaign, there was light relief as thousands of voters brought their pugs, poodles and other pets to the polling stations, posting their pictures on Twitter using the hashtag #dogsatpollingstations.

A final poll by YouGov on Wednesday put the Conservatives seven points ahead of Labour, while ICM gave May's party a 12-point lead over its rivals.

Such predictions stand in stark contrast to a Monday poll by Survation, which gave a narrow one-point gap.