Britain's electoral commission has promised what it says will be a full investigation after queues of angry would-be voters were turned away from polling stations in the UK election.
In one case a polling station in the northern city of Liverpool even stopped accepting voters for about an hour after they ran out of ballot papers.
The incidents at several polling stations lead to suggestions that thousands of voters had effectively been disenfranchised.
Election officials admitted there would likely be legal challenges to some results in what was set to be a cliffhanger national election.
In some cases police were called in to calm crowds of voters unable to cast their ballots after a number of polling stations closed with long queues of people still outside.
Some would-be voters reportedly tried to prevent officials from taking away ballot boxes in protest at being denied access to vote.
In Sheffield in the north of England, about 200 people were turned away from a polling station when polls closed at 21:00 GMT, while a similar number were reportedly turned away from another polling station in Manchester.
Other problems were reported in the constituencies of Islington south, Newcastle east, Liverpool and Lewisham, while police said they were called to a protest at a polling station in Hackney, northeast London.
Marcus Fairs, a voter in London's Stoke Newington, said he saw some 30 people being turned away from his local station.
He told Al Jazeera that the reaction "was one of anger and disbelief".
"Half of those turned away complained with the officers that they were there before 10pm," he said, adding "it was like a mini-riot".
'We got this wrong'
Pictures and videos posted on social networking sites Youtube and Twitter showed police ushering crowds of people away in what is being seen as the UK's closest election in decades.
"People want to vote, so when people aren't allowed to vote obviously they aren't going to be very happy"
Robin Dallman, a postman in Sheffield, told the Reuters news agency he arrived at the polling station 30 minutes before closing time, but failed to reach the door.
"People want to vote, so when people aren't allowed to vote obviously they aren't going to be very happy," he said.
John Mothersole, a returning officer for the city, apologised to voters who were prevented from voting saying "we got this wrong".
He told the Press Association that a number of voters, mainly students, were turned away because they did not have polling cards.
"This made the administration process of ensuring the correct person was given a ballot paper much longer," Mothersole said.
Britain's electoral Commission said challenges to some results were likely.
Jenny Watson, head of the commission, told the BBC that it would undertake a "thorough review" of the problems, and acknowledged that there may need to be a change in the law to redraw the rules.
|In one case a polling station turned away voters after running out of ballot papers [AFP]
"Clearly, parliament will want to look at this and it may well be that the law will need to change," she said.
A spokesman for the opposition Conservative party said reports of voters being turned away were "disturbing", while Tessa Jowell, Britain's Olympics minister, said the incidents amounted to a "scandal".
"These are queues of people exercising their democratic right and then being denied it," she said.
However in Lewisham, a constituency in London's south, polls were kept open for an extra 30 minutes to cope with "large queues" still waiting to vote at the deadline.
This could also pose a challenge to British electoral rules, which state that people must have been issued with a ballot paper by 10pm local time (21:00 GMT) to be allowed to vote.
If they are too late, they are not permitted to vote.