London’s police service said Sunday that officers had arrested 522 people the previous day for breaching anti-terror laws by supporting the recently proscribed group Palestine Action.

In an update to its previous arrest tally, the Met said all but one of those 522 arrests took place at a Parliament Square protest and were for displaying placards backing Palestine Action.

The other arrest for the same offense took place at nearby Russell Square as thousands rallied at an anti-Israel demonstration organized by the UK’s Palestine Coalition.

The 522 arrest total is thought to be the highest ever recorded at a single protest in the UK capital.

The Met made 10 further arrests, including six for assaults on officers, though none was seriously injured, it added.

The force said the average age of those arrested on Saturday was 54, with six teenagers, 97 aged in their 70s and 15 octogenarians. A roughly equal number of men and women were detained.

The government outlawed Palestine Action on July 5, days after it took responsibility for a break-in at an air force base in southern England that caused an estimated £7 million ($9.3 million) of damage to two aircraft.

The group said its activists were responding to Britain’s indirect military support for Israel, during the war in Gaza.

Britain’s interior ministry has said Palestine Action was also suspected of other “serious attacks” that involved “violence, significant injuries and extensive criminal damage.”

In a statement following the latest mass arrests, British Interior Minister Yvette Cooper defended the government’s decision, saying: “UK national security and public safety must always be our top priority.”

“The assessments are very clear — this is not a non-violent organization,” she added.

Critics including the United Nations and groups such as Amnesty International and Greenpeace have condemned the proscription of Palestine Action as legal overreach and a threat to free speech.

“If this was happening in another country, the UK government would be voicing grave concerns about freedom of speech and human rights,” Greenpeace UK’s co-executive director Areeba Hamid said Saturday.

She added the government had “now sunk low enough to turn the Met into thought police, direct action into terrorism.”

Police across the UK have made scores of similar arrests since July 5, when being a member of Palestine Action or supporting the group became a criminal offense punishable by up to 14 years in prison.

Police announced this week that the first three people had been charged in the English and Welsh criminal justice system with such backing following their arrests at a July 5 demo.

In its update Sunday, the Met revealed a further 26 case files following other arrests on that day are due to be submitted to prosecutors “imminently” and that more would follow related to later protests.

It believes 30 of those held Saturday had been arrested at previous recent Palestine Action protests.

Eighteen people remained in custody Sunday lunchtime, but were set to be bailed out within hours, the Met added.

It said officers from its counterterrorism command will now “work to put together the case files required to secure charges against those arrested as part of this operation.”