Zoe, one of those taking part in the demonstration, said: “I along with hundreds of ordinary people have had enough of our government’s collusion in genocide.

“You can hear the claps and it’s got like a family-friendly environment here. You’ve seen it with your own eyes. It’s completely peaceful and there’s nothing vaguely terrorist right now. We’re just being ordinary peaceful people saying we oppose genocide,” she said.

She wore a large patch with the words “Jews Against Genocide”. She said: “I wear it clearly and with pride to make sure that other people recognise that there are many many Jews who do not support genocide of any nature.”

“I’m holding a piece of paper for goodness sakes, that’s not a terrorist act. A terrorist act is intentionally starving 2 million,” she said. “I don’t know what to say to [Palestinians], I’m so sorry it’s come to this. I wish we could do more but we’re trying our best.”

Officers searched the bags of those arrested. In one backpack handled with blue forensic gloves, they uncovered some bread and a milk carton filled with water.

Robert Del Naja, from the band Massive Attack, joined the sign-holders and said: “UK civil liberties are trapped in a manufactured crisis. Peaceful citizens of conscience — including pensioners — have become terrorists, at the will of a human rights lawyer turned authoritarian who now lunges at opinions that expose the moral vacuum of his unrecognisable government.”

Some of those arrested were publicly processed on the street outside Scotland Yard, near the main demonstration, where crowds gathered and shouted “‘shame on you” at officers.

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.”