More than 450 people have been arrested in central London at the largest demonstration relating to Palestine Action since the group was proscribed as a terrorist organisation.

The Metropolitan police said they had drawn officers from other forces to help form a “significant policing presence” in the capital before the planned event, which was anticipated to result in large numbers of people being detained.

By Saturday afternoon, hundreds of people had gathered in Parliament Square for a demonstration organised by the campaign group Defend Our Juries, who said “approximately 1,000 sign-holders” had turned up.

The Met said it estimated 500 to 600 people were in Parliament Square when the demonstration began, but “many” were not partaking. On Saturday night, it said: “Parliament Square and Whitehall are clear. As of 9pm, 466 people had been arrested for showing support for Palestine Action.

“There were a further eight arrests for other offences including five assaults on officers.”

A spokesperson for Defend Our Juries said earlier: “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.”

On Saturday evening, the home secretary thanked officers and reaffirmed the government’s decision to ban Palestine Action.

“Many people may not yet know the reality of this organisation, but the assessments are very clear – this is not a nonviolent organisation. UK national security and public safety must always be our top priority,” Cooper said.

The demonstration began at 1pm but officers made a small number of arrests beforehand. Soon after, officers began arresting people outside the green holding signs that referenced Palestine Action. One woman, while being dragged away by police, said: “All of you have loved ones, you’re arresting the wrong people. Arrest the people over there [pointing towards parliament], arrest the people complicit in the genocide.”

From 1pm to 2pm, demonstrators holding signs that read “I oppose genocide, I support Palestine Action” were silent. “It’s all very sedate, peaceful and moving,” said Suzan Broom, who clapped and cheered for those being taken away by police. “I’ve been following them and saying thank you because I’m too scared.”

Two women taking part in the action, one in her mid-70s, were arrested by police midway through their interview with the Guardian. One said of the turnout: “I think it’s great, people are absolutely sick of seeing genocide on their screens.”

She said the vandalism of RAF aircrafts by Palestine Action members was a “protest action, it’s not at all terrorism”.

“Those planes are the terrorists. They’re going over and shooting and killing children,” she said.

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.”

Before the protest, a Home Office spokesperson said: “The home secretary has been clear that the proscription of Palestine Action is not about Palestine, nor does it affect the freedom to protest on Palestinian rights.

“It only applies to the specific and narrow organisation whose activities do not reflect or represent the thousands of people across the country who continue to exercise their fundamental rights to protest on different issues. Freedom to protest is a cornerstone of our democracy and we protect it fiercely.”

The first three people to be charged with supporting the group in England and Wales were named on Thursday.

Jeremy Shippam, 71, Judit Murray, 71, and Fiona Maclean, 53, have been charged with displaying an article in a public place, arousing reasonable suspicion that they are a supporter of a proscribed organisation after their attendance at a previous demonstration last month.

Palestine Action was banned in June, days after activists from the group broke into RAF Brize Norton and defaced two military aircraft with spray paint. The ban means that membership of, or support for, Palestine Action is a criminal offence punishable by up to 14 years in prison under the Terrorism Act 2000.

In a separate development, the government announced on Saturday it would send an additional £8.5m of aid to Gaza via a UN agency, a day after Keir Starmer said Israel’s expansion plans were “wrong” and “will only bring more bloodshed”.