Half of the people arrested during the protest in relation to Palestine Action in London on Saturday were aged 60 or above, according to police figures.
A total of 532 people were arrested at the largest demonstration relating to the group since it was proscribed last month. All but 10 were under Section 13 of the Terrorism Act for displaying supportive placards or signs.
An age breakdown released by the Metropolitan police on Sunday revealed that nearly 100 of those detained were in their 70s and 15 were in their 80s. Of the 519 arrested people with confirmed dates of birth, 49.9% were 60 or older.
Hundreds attended the event in Parliament Square organised by Defend Our Juries, which asked participants to hold up signs saying: “I oppose genocide. I support Palestine Action.”
Among those arrested was Sir Jonathon Porritt, 75, a former government adviser who said he had long been concerned about the erosion of civil liberties under successive governments. He was arrested under Section 13and bailed until 23 October
He described the proscription of Palestine Action as “a measure of the government’s desperation” and “entirely inappropriate”.
“I thought this was overreach by the home secretary, trying to eliminate the voices of those who are deeply concerned about what is happening in Gaza,” he said.
“This was an absolutely clear case of a government using its powers to crush dissenting voices when it is the government itself that is most reprehensible for what continues to be an absolute horror story in the world.
“What we are seeing now in Gaza has just utterly shocked people and it’s completely abhorrent that we are living through a genocide on our TV screens.”
Porritt said he had no complaints about how he had been treated, describing officers as courteous. Some, however, said older people who had been arrested were denied access to water and were kept waiting for long periods in the sun to be processed.
The Met said it had taken precautions before Saturday’s operation on the basis of its duty of care and because of the age of those who had been arrested at previous protests. It also said the decision to turn out was a question of “personal responsibility”.
“There was water available at the prisoner processing points and access to toilets. We had police medics on hand as part of the policing operation and we processed people as quickly as possible to ensure nobody was waiting an unreasonably long time,” the force said.
“Notwithstanding that, a degree of personal responsibility is required on the part of those who chose to come and break the law. They knew they were very likely to be arrested which is a decision that will inevitably have consequences.”
Others arrested include Chris Romberg, 75, a former British army officer colonel and military attache at UK embassies in Egypt and Jordan, and the son of a Holocaust survivor. Like Porritt, he was arrested under Section 13 and bailed.
He said: “This is a serious assault on our freedoms.
“When I protested against the US war in Vietnam, we were able to chant ‘victory to the NLF’ without being criminalised. Now a statement of support for a non-violent direct action group is prosecuted under anti-terrorism legislation.”
He had no complaints about police treatment.
Alice Oswald, 58, an award-winning poet, said she had urged police who detained her to write to the home secretary about the position they had been put in.
“Clearly there were some police officers who were really struggling with what they had to do,” she said. “You could see the slightly shifty look in their faces, too. When I was speaking to them in the police van I did say: ‘Write to Yvette Cooper and tell her that this is making your life impossible.’”
Oswald, who won the TS Eliot prize in 2002 and was professor of poetry at the University of Oxford, said her motivation for taking part included the very personal experience of giving online poetry classes regularly to young people and children in Gaza.
She said she had been arrested under Section 12 of the Terrorism Act, which relates to inviting support for a proscribed organisation.
Of the 532 arrests, six people were detained for allegedly assaulting police officers after large numbers protesters gathered in central London for a separate march organised by the Palestine Solidarity Campaign and others.
One arrest was for obstructing a constable in the execution of his or her duty, two were for breaching Section 14 Public Order Act conditions, and one for a racially aggravated public order offence. Eighteen people were still in custody by 1pm on Sunday but were expected to be released hours later.
In the age breakdown of arrests, the Met said the dates of birth of 13 people awaited clarification and were not included.
On that basis, the average age of those arrested was 54. Males accounted for 263 of those detained, females 261 and eight either defined themselves as non-binary or did not disclose their gender.
Ten people have been charged across the UK for suspected offences under the Terrorism Act since the proscription of Palestine Action, authorities said last week.
The home secretary proscribed Palestine Action last month after activists caused an estimated £7m of damage to jets at the RAF Brize Norton military base in Oxfordshire.
Cooper has said the decision was based on strong security advice and followed what she described as “disturbing information referencing planning for further attacks,” the details of which cannot yet be publicly reported because of ongoing legal proceedings.
