There have been length queues outside polling stations
Hundreds of voters have been turned away by polling stations across London, some saying they were understaffed.
Election ballots are being counted for 73 parliamentary constituencies in the city and all 32 borough councils. Polls closed at 2200 BST.
A Met Police spokesman described queues of 300 "disappointed people" unable to vote in Lewisham and 150 in Hackney.
There were three-hour queues in Sheffield and problems reported in Manchester, Liverpool and Birmingham.
Turnout was predicted to be higher than recent elections, including 2005.
BBC London Political Editor Tim Donovan said the problems may turn out to be one of the night's biggest stories.
Harriet Harman, Labour deputy leader and candidate for Camberwell, south London, said it was "quite right" that results in affected areas could be challenged.
She said: "If there is any close outcome that is going to be produced by that there should be a legal challenge.
"It is fundamental that people get their right to vote."
Liberal Democrat leader Nick Clegg has apologised to voters who have been waiting in long queues at a polling station in his Sheffield constituency.
Asked whether it amounted to a "scandal", Minister for London Tessa Jowell replied: "I think it is. These are queues of people exercising their democratic right and then being denied it."
Andrew Boff, Conservative mayoral candidate in Hackney, said it was "getting ugly" after people were told they could not vote at a polling station with just three staff.
Voters at the Hackney polling station staged a sit-in protest at being unable to vote.
Some are reported to be heading to the count at Hackney Town Hall to voice their displeasure.
Georgia Ladbury visited her polling station in Vauxhall three times before being able to vote and said people were being turned away by 2200 BST.
She said: "It was not as if everyone turned up at 9.55pm expecting to vote.
"The station was definitely understaffed and that would seem to be the only explanation for the problems."
Police officers at a polling station in Brockley, south London, told the BBC that voters were still being allowed to vote at 2230 BST.
The Electoral Commission said if people had been issued their ballot paper by 2200 BST they were still legally able to vote after that time.
It promised a "thorough review" of what had happened in constituencies where people were unable to vote.