Voters have been pictured queuing outside polling stations, with long lines of people snaking around in a line in some places.
Pictures from South London have shown large queues in Clapham, Streatham, Balham and Battersea.
There were also queues in Bermondsey in South East London, reports the Metro.
In Greater Manchester, long queues of voters were seen in Ancoats first thing on Thursday as the polling stations opened.
Rebecca Grant, who was at Ancoats, told the Manchester Evening News: "I went to vote this morning and there were substantial queues outside the Victoria Square polling station on Oldham Road, Ancoats.
"My flatmate went after me (8:30am) and said the situation was much worse - he estimated about 100 people and queues standing outside the building."
It is unclear if it will have any significant impact on the outcome of Thursday's general election - or if it is reflective of the rest of the country.
The Times' deputy political editor, Steven Swinford, tweeted: "Big queue of 100+ people ahead of me at the polling station in marginal Battersea.
"There's 50 odd people behind me too."
Aidan Conway in Balham said he had never seen a queue that big during an election in the Tooting constituency.
He tweeted: "I have been living in Balham, London, for six years. I have never seen a queue like this at my polling station."
Martin Archer had been waiting for 45 minutes in a large queue in Streatham and he said he wasn't even near to the front.
"I've never seen such queues at the polling station before."
Many of the voters said the lines were "full of young people".
In Jeremy Corbyn's Islington constituency, Lebby Eyres said: "Today was the first time I have ever had to queue for any election, general, council or the referendum.
"Squeaky bum time."
A couple in Merseyside claimed they had been turned away from a polling station without being able to vote.
Ellis Bennett, 18, said he was shocked to be turned away from the polling station in Woolton Village, after registering to vote three weeks ago.
The polling stations close at 10pm tonight and 650 MPs will be chosen in the first-past-the-post system.
An exit poll will be published as soon as the polls have closed, which will give a snapshot of the result. The individual constituencies' results will come in through the night as the counts take place.
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