Palestine Action has been proscribed since 5 July, making it a criminal offence to show support for the organisation, but police said a “significant number” were carrying placards backing the group.
Amnesty International says it is “deeply concerning” that police made 474 arrests during a Palestine Action demonstration in London.
Metropolitan Police said 466 were detained under the Terrorism Act for showing support for a banned group.
Eight more people were arrested for other offences, including five for assaulting officers.
The Met said it was the most arrests it’s made related to a single operation in at least the past decade.
Organiser of the event, Defend Our Juries, earlier said up to 700 people were at the event in Parliament Square and claimed police were preparing for the “largest mass arrest in their history”.
The group said those arrested included former Guantanamo Bay detainee Moazzam Begg, NHS workers, quakers and a blind wheelchair user.
Amnesty International UK’s chief executive Sacha Deshmukh said in a statement: “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.
“Instead of criminalising peaceful demonstrators, the government should be focusing on taking immediate and unequivocal action to put a stop to Israel’s genocide and ending any risk of UK complicity in it.”
The Met said a “significant number of people” at the event were seen “displaying placards expressing support for Palestine Action”.
Home Secretary Yvette Cooper said: “The right to protest is one we protect fiercely but this is very different from displaying support for this one specific and narrow, proscribed organisation.
“Palestine Action was proscribed based on strong security advice following serious attacks the group has committed, involving violence, significant injuries and extensive criminal damage.”
Police said those arrested had been taken to processing points in Westminster and any whose details could be confirmed were bailed on condition they didn’t attend further Palestine Action support events.
Others whose details could not be verified, possibly because they refused to give them, were taken to custody suites across London.
The protests have put a strain on authorities’ capacity to cope.
Sky News understands senior leaders in the prison service, known as “Capacity Gold”, met today to discuss how to deal with the large number of arrests as the male prison estate is close to full.
It’s understood 800 inmates were moved out of the busiest jails in and around London beforehand.
In a statement, the Metropolitan Police Federation said: “Thinking of our colleagues and wishing all assaulted officers well. Remember there are no ‘extra’ police officers – just the same ones having their days off cancelled, having to work longer shifts and being moved from other areas. Officers are emotionally and physically exhausted.”
Legislation to ban Palestine Action came into force on 5 July, making it a criminal offence to show support for the organisation, carrying a prison sentence of up to 14 years.
Defend Our Juries said earlier this week the protest would still go ahead, following several similar demonstrations since it was outlawed last month.
On Saturday, a spokesperson said: “Palestine Action and people holding cardboard signs present no danger to the public at large.”
Human rights advocates Amnesty International described the arrests of so many people under UK terrorism law as “deeply concerning”.
Another march organised by the Palestine Coalition, which is a separate group, set off from Russell Square and assembled on Whitehall.
The Met Police said one person had been arrested there for showing a placard in support of Palestine Action.
