Miles Pickering, an engineer from Brighton, was among 532 people detained at the protest. He said he was arrested under section 13 of the Terrorism Act 2000, which makes it an offence to wear anything supporting a proscribed organisation.

Mr Pickering insisted his clothing did not reference Palestine Action – banned in July after activists admitted breaking into RAF Brize Norton – but instead displayed a message opposing “AI-generated animation”. The T-shirt showed the stop-motion character Morph giving a thumbs up from inside the letter ‘O’.

He later told The Guardian an officer had glanced at his top and said “right, you’re nicked” before taking him to Scotland Yard, where he was photographed. “They were taking photos of me, and everyone was laughing at how silly it was that I was getting arrested for being a plasticine terrorist,” he said.

Mr Pickering claimed a senior officer even raised the possibility of charging him under section 12 of the Terrorism Act, but the arresting officer eventually intervened after checking the wording of his shirt. “He said: ‘I’ve got good news and I’ve got bad news.’ I said: ‘What’s the good news?’ He said: ‘I’m de-arresting you.’ And I said: ‘What’s the bad news?’ He said: ‘It’s going to be really embarrassing for me.’”

Mr Pickering has since begun selling the shirts to raise funds for Medical Aid for Palestinians and hopes to see them worn at the next protest against the ban on Palestine Action on 6 September. “If 1,000 people wander around the Palestinian march wearing a T-shirt that says Plasticine Action with a picture of Morph, what are they going to do?” he said.

More than 700 people have been arrested for supporting Palestine Action. The Metropolitan Police said more than 100 of those detained on 9 August were over 70, with the average age of those arrested being 54. The force was contacted for comment.

A viral video from a pro-Palestinian demonstration in Glasgow that took place on Saturday shows officers detaining a man. They did not, however, arrest him.