Dozens of EU citizens gathered outside Parliament seeking guarantees their rights will not be diluted post-Brexit.
Campaigners called for a "free and simple registration process" ensuring the 3.2 million EU citizens in the UK and 1.2 million British expats in the EU retain their current status for life.
Nicolas Hatton said Europeans felt their rights were "under threat" and he had organised a "big day of action" in response. The Frenchman, who has lived in Britain for 22 years, founded the3million, a group representing EU nationals in the UK.
He said it is "wrong to move the goalposts after the match has started for those of us who are already living and working here".
Mr Hatton added: "We came to Britain and made it our home. We paid taxes, we started businesses, we started families. All we want is exactly what we have now. We're not asking for anything new or anything special."
They reject the "settled status" option outlined by the Prime Minister, arguing it would strip them of equal family reunification rights and the ability to leave the UK and return after two years.
The activists want their rights guaranteed for life under the European Court of Justice (ECJ) to prevent a future government rolling back assurances.
But an early agreement has not been possible as the UK and EU have fundamental disagreements on the role of the ECJ.
Business consultant George Chiesa, 59, said members of society who "abide by the law don't have to prove they're innocent".
He said providing documents proving he has lived in Britain stretching back 25 years would be "impossible, absolutely impossible".
The Italian national said there was a danger "we will be deprived of the right to live in the place we call home."