Pollsters YouGov on Wednesday gave the Conservatives a seven-point lead over Labour, while ICM put Theresa May's party 12 points ahead.
Britain went to the polls on Thursday for a snap election called by Prime Minister Theresa May ahead of Brexit after a campaign shadowed by terrorism.
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 take Britain into Brexit talks.
But 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. (AFP)
Opposition Labour leader 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.
Corbyn, a 68-year-old leftwinger 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.
"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. (AFP)
It is the third time Britain has gone to the polls in two years, twice for a general election and once for the EU referendum, and voter fatigue appeared to be an issue among the early voters.
"I don't think it has really been a campaign, we don't know anything about what they are going to do about Brexit, it's been pointless really," said Joe Kerney, 53, at a polling station in Hackney, east London.
"I have little confidence in anybody," added voter Simon Bolton, 41. "I think we lack quality in terms of who we can choose, it is very limited."
Security on voting 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.
Polling stations, many located in schools and community centres, opened at 7:00am (0600 GMT) and will close at 10:00pm, with 49.6 million registered voters electing a total of 650 MPs to parliament.
An exit poll will give an indication of the outcome, although final results will not emerge until early Friday.