London police arrested hundreds of protesters for supporting a banned pro-Palestinian group at a demonstration on Saturday which went ahead despite requests to call it off after a deadly attack at a synagogue in Manchester.

Two people were killed in the attack in the northwestern English city on Thursday and police shot dead the assailant, a British man of Syrian descent who counter-terrorism police said may have been inspired by extremist Islamist ideology.

Organisers refused requests by the police and government to call off Saturday's demonstration, which had been announced before the attack, to protest against the banning of pro-Palestinian group Palestine Action under anti-terrorism laws.

Calling for calm on X on Saturday morning, British Prime Minister Keir Starmer said: "I urge anyone thinking about protesting this weekend to recognise and respect the grief of British Jews."

"This is a moment of mourning. It is not a time to stoke tension and cause further pain. It is a time to stand together," he said.

Police arrested protesters in Trafalgar Square in central London as they wrote slogans on placards declaring support for Palestine Action, which was proscribed in July after members broke into an airbase and damaged military planes.

Crowds of more than 1,000 applauded and cheered those arrested as they were carried through the crowd by police without resisting. Onlookers chanted "shame on you" at officers.

The demonstration is the latest in a series of protests, during which hundreds have been arrested for defying the ban which makes it an offence to show support for Palestine Action. (Reuters)