London (AFP) - Rights groups representing Britons living in Europe and EU citizens in Britain formed a human chain outside Downing Street on Monday, demanding the government guarantee their statuses in a no-deal Brexit scenario.

The protesters, who linked arms along Whitehall in the heart of London's political district, then delivered a letter to Prime Minister Theresa May saying they were "extremely concerned" at stalled Brexit negotiations.

"No deal is not an option for EU citizens," Nicolas Hatton, the co-founder of lobby group the3Million which represents EU nationals living in Britain, told AFP ahead of handing the two-page letter in at Downing Street.

"We just want fairness in the way we are treated," he said.

The message -- from the3Million, the British in Europe coalition and trade union Unison -- called on May "to honour the political promises made to us, no matter what the outcome of Brexit negotiations".

"We are not bargaining chips, we are five million people and it is time we were treated as such," it said.

The letter also called for strengthening their rights in areas like voting.

"The loss of our voting rights is one of the most undemocratic and shameful aspects of Brexit," it added, noting at least 60 percent of Britons in Europe and almost all EU citizens were disenfranchised in the 2016 referendum.

- 'Second class citizens' -

Advocate groups for overseas citizens directly impacted by Brexit have long called for their rights to be ring-fenced from the rest of the negotiations.

With less than five months to go until Britain's scheduled departure from the bloc, and the talks at an impasse over the Irish border, they are increasingly anxious.

"I worry about it because you don't know what's happening," said Vanessa Verlinden, 42, protesting draped in the flag of her native Belgium but who has lived in Britain 17 years.

"I'm not making any plans for 2019, because we just don't know where we'll be -- it's up in the air," she added.

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Karen Lawrance, 52, an engineer and pro-Europe activist from Worcestershire in central England, had travelled to the capital to attend several Brexit-related events this week.

"Why should these people be treated as second class citizens? It's absolutely wrong," she told AFP.

Attendees, wearing hats stitched with the stars of the EU flag and badges with the slogan "I am not a bargaining chip", will also lobby lawmakers inside parliament later Monday.

The government will face an urgent question on the issue in the House of Commons tabled by pro-Remain opposition Labour MP Yvette Cooper.

"We're there to tell MPs we need a deal," added Hatton. "We want our rights to be separate."

The 48-year-old said weekend reports that a Brexit deal between Brussels and London could be imminent did little to allay his fears.

"It's very difficult because every week there's a different interpretation," he added of the on-going talks.