Two EU citizens' rights groups have said they will be bringing forward a legal challenge after thousands of voters reported being turned away from polling stations in the UK on Thursday.

British in Europe and the3million sponsored a crowdfunding campaign which had already achieved its £20,000 (€22,692) goal within four hours of going live and donations kept flooding in.

The groups, who represent EU citizens in the UK and British citizens abroad, plan to explore a legal challenge that could "potentially lead to compensation for those who weren't allowed to vote."

They have provided a complaint form for those who were denied their vote to signal it to the Electoral Commission. Over 3,500 people used the form on May 23.

Many took to Twitter to express their support using the hashtag #Deniedmyvote.

It's a hashtag the groups said appeared over 100,000 times on May 23 when the UK held EU parliamentary elections. Indeed it was already trending Thursday morning just hours after polls opened.

EU citizens living in the UK were often denied a vote because they needed a voter registration form stating that they would not vote in their home country, a bureaucratic hoop they may not have known about.

The chief executive of the Electoral Commission, Bob Posner, blamed in part the UK government's whiplash decision to hold elections after delaying Brexit in an article published in The Guardian.

"These are some of the reasons why we and the wider electoral community raised concerns with the government, with rising urgency in the first months of this year," Posner wrote, stating that European parliament elections do not follow the same "well-trodden path" as a general election.

"Only on 7 May – the date by which voters had to be registered to vote at the poll – did the government confirm that it would go ahead," Posner continued. This, he said, put staffing and administrative pressures on the commission.

He pointed out that the electoral commission needs a process "to guard against people voting in two countries" but that updating the rules would require changes to the electoral law.

The citizens' rights groups are bringing the case to the courts.

"Imagine being turned away at the polling station in elections you always voted in?" asked Nicolas Hatton, co-founder of the3million, a group representing EU citizens in Britain.

"We should not have been treated like second class citizens and we are calling those who believe in fairness to support us challenge the Government in court over the disenfranchisement, discrimination and disrespect of EU citizens in the UK and British citizens in the EU," he said.