(CNN)Hundreds of EU citizens living in Britain have been turned away at polling stations as they attempted to vote in the European elections on Thursday, campaigners say.
Voters reported confusion with the process for participation in the elections, failings by local councils, and names being crossed off voting lists at polling stations.
Hours after the polls opened, the hashtag #DeniedMyVote began to trend on Twitter as European nationals began reporting they had been turned away.
Campaigners fear the number of EU nationals denied a vote will be in the thousands by the time polls close on Thursday evening.
Maike Bohn, the co-founder of the3million, a campaign group which represents EU nationals living in the UK, told CNN the situation reflects "a serious democratic deficit" on the part of British authorities.
EU nationals who live in the UK can participate in European elections but must make a written declaration, known as a UC1/EC6 form, that guarantees they will not vote in their home country.
It appears that the unplanned delay to the Brexit process has contributed to the confusion. Britain's participation in the 2019 European elections was only confirmed last month, after the country's failure to agree a deal to leave the EU by the original March 29 deadline.
Campaigners accuse British authorities of failing to inform EU citizens that they had to complete an extra step to guarantee their right to vote.
A number of Members of Parliament (MPs) expressed alarm over the reports. Joanna Cherry of the Scottish National Party said on Twitter: "There's something very rotten at the heart of British politics when EU citizens resident here are being denied their right to vote."
The Labour MP and shadow foreign minister Catherine West called it a "national scandal."
"Reports are flooding in of EU nationals being denied the right to vote in today's elections, despite the fact most of them were registered to vote in local elections that took place a few weeks ago. I was granted an urgent question back in April, at which I alerted the government to this problem -- they refused to act."
Three EU nationals who spoke to CNN told of confusion over the process for participating in the poll.
Swedish citizen Lina Dencik, who lives in southwestern city of Bristol, told CNN that she was turned away from the polling booth because the documentation had not reached her before the May 7 voter registration deadline for EU citizens.
"When I went to the polling station this morning, my name had been crossed out on the list and I was told I was not eligible to vote as a European citizen," she said.
"I challenged this and the patrolling officer provided me with an emergency number to call. I called this number and was told that a form to declare that I would not be voting in Sweden had not been sent out in time and that I am therefore not able to vote."