Amnesty International called the mass arrests “deeply concerning”.

“The protesters in Parliament Square were not inciting violence and it is entirely disproportionate to the point of absurdity to be treating them as terrorists,” said Sacha Deshmukh, the organisation’s chief executive.

“We have long criticised UK terrorism law for being excessively broad and vaguely worded and a threat to freedom of expression. These arrests demonstrate that our concerns were justified.”

Amnesty International called the mass arrests “deeply concerning”.

“The protesters in Parliament Square were not inciting violence and it is entirely disproportionate to the point of absurdity to be treating them as terrorists,” said Sacha Deshmukh, the organisation’s chief executive.

“We have long criticised UK terrorism law for being excessively broad and vaguely worded and a threat to freedom of expression. These arrests demonstrate that our concerns were justified.”

The Met said it estimated 500 to 600 people were in Parliament Square when the demonstration began, but “many” were not partaking. By 6pm, the force said 365 people had been arrested.

A spokesperson for Defend Our Juries said: “The fact that unprecedented numbers came out today risking arrest and possible imprisonment shows how repulsed and ashamed people are about our government’s ongoing complicity in a livestreamed genocide, and the lengths people are prepared to go to defend this country’s ancient liberties.”

“The police have only been able to arrest a fraction of those supposedly committing ‘terrorism’ offences, and most of those have been given street bail and allowed to go home. This is a major embarrassment to Yvette Cooper, further undermining the credibility of this widely ridiculed law.”