The number of arrests at Saturday’s demonstration in London in support of banned group Palestine Action has risen to 532.

The Metropolitan Police said the majority of arrests – 521 – were for displaying placards in support of Palestine Action at Westminster’s Parliament Square, and one at a Palestine Coalition march.

Six arrests were for assaulting police officers, two for breaching Public Order Act conditions, one for obstructing a constable and one for a racially aggravated public order offence.

It was the biggest protest since the government proscribed the group in July under the Terrorism Act, making membership of or support for it a criminal offence, punishable by up to 14 years in prison.

The force previously said that the number of arrests was the largest made by the force on a single day in the last 10 years.

The average age of those arrested was 54, and the most arrests – 147 of them – were of people aged between 60 and 69.

The Metropolitan Police said its counter terrorism team was now working on bringing charges against those accused of supporting Palestine Action.

“Over the coming days and weeks, officers from the Met’s Counter Terrorism Command will work to put together the case files required to secure charges against those arrested as part of this operation.”

The protest, organised by Defend Our Juries, saw hundreds of people simultaneously pull out placards with the message “I oppose genocide. I support Palestine Action” when Big Ben chimed at 13:00 BST on Saturday.

Police started making multiple arrests at a time, working their way through the crowds, with those who refused to move being carried out by police to chants of “shame on you” directed at the officers.

Protesters whose details could be confirmed at temporary prisoner processing points nearby were released on bail to appear at a police station at a future date, under condition not to attend future demonstrations related to Palestine Action.

But 212 protesters who refused to provide their details or were found to have been arrested while already on bail were taken into custody.

Police said as of 13:00 on Sunday, 18 of them remained in custody but were expected to be released during the day.

Security expert Lizzie Dearden told the BBC that the laws the protesters were arrested under “were not designed for a group of this nature or a group of this size”.

“When [the Terrorism Act] was drafted in the late Nineties, the kind of terrorist group that was really being envisaged were groups like the IRA, al-Qaeda, armed militant groups that were operating often abroad.

“So the law is being applied in a completely new way.”

Home Secretary Yvette Cooper thanked police for their response following the demonstration, and said the banning of Palestine Action was based on “strong security advice following serious attacks the group has committed” as well as “plans and ideas for further attacks”.

“Many people may not yet know the reality of this organisation, but the assessments are very clear – this is not a non-violent organisation,” she said.

But charity Amnesty International’s chief executive Sacha Deshmukh described the mass arrests as “deeply concerning”.

He said: “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.”

At the end of July, the High Court ruled that Palestine Action would be able to challenge its proscription.

Lawyers for the group’s co-founder Huda Ammori have argued that the ban breaches the right to free speech and has acted like a gag on legitimate protest.

The government says the ban is justified because it narrowly targets a group that has been organising serious criminality.