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Students tried to prevent ballot boxes from being taken away for the count after they were turned away from a polling station.
There were angry scenes at the polling station in Sheffield Hallam, where Liberal Democrat leader Nick Clegg was standing, after dozens of voters were turned away. Police were called and Sheffield's deputy returning officer Lee Adams admitted staff "couldn't cope" with the number of people who turned up at Ranmoor polling station.
The founder of a protest group on a social networking website said students were segregated from other voters and she accused polling staff of "discrimination".
Sheffield University student Kate Baldwin, 19, who set up the Facebook group, said she queued for two hours to vote at the St Johns polling station, along with hundreds of others. Miss Baldwin said there was anger when election officials divided the queue into "residents" and students.
John Mothersole, returning officer for Sheffield, said: "We got this wrong and I would like to apologise. We were faced with a difficult situation with the numbers of people, and a large amount of students turning up to vote without polling cards. This made the administration process of ensuring the correct person was given a ballot paper much longer."
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Meanwhile, voters staged a sit-in protest after a London polling station closed with queues of people outside. Hackney's two Labour parliamentary candidates launched an official complaint over their supporters' inability to vote in the east London seats. At least 150 people were waiting when the polls closed, according to Andrew Boff, Conservative mayoral candidate. He estimated the number unable to vote could have been double that, as some had given up in the face of the long queues.
A Hackney Labour spokesman said some of the party's "life-long supporters" were barred from voting. "Hackney's two Labour candidates to become MP, Diane Abbott and Meg Hillier, launched an official complaint to the borough's returning officer tonight after Labour candidates relayed protests from life-long Labour supporters that their votes had been ignored," he said.
Conservative Party leader David Cameron said a new government must ensure there was no repeat of chaotic election day scenes in which people were still queuing in their hundreds outside polling stations when the ballot boxes closed.
The Electoral Commission pledged a "thorough review" into problems around the country, including London, Sheffield, Manchester, Liverpool and Newcastle.
Labour's deputy leader Harriet Harman said it would be "quite right" if some results were challenged because voters were turned away without being able to cast their ballot. Branding the situation as "outrageous" Ms Harman said: "If there is any close outcome that is going to be produced by that there should be a legal challenge - and quite right too."
