More than 425 people were arrested in London on Saturday during a tense protest in support of the Palestine Action group, which has been banned under terror laws, British police said.

Britain banned Palestine Action under anti-terrorism legislation in July after some of its members broke into a Royal Air Force base and damaged military planes.

The group, which has also targeted defence firms in Britain with links to Israel, accuses Britain's government of complicity in what it says are Israeli war crimes in Gaza.

Police have arrested hundreds of Palestine Action supporters in recent weeks under anti-terrorism legislation, including more than 500 in just one day last month, many of them over the age of 60.

Hundreds of demonstrators gathered near parliament in central London on Saturday to protest against the ban, with many holding up signs that said: "I oppose genocide. I support Palestine Action."

London's Metropolitan Police said people had been detained for a number of offences including assaulting police officers and supporting a proscribed organisation.

"Officers ... have been subjected to an exceptional level of abuse including punches, kicks, spitting and objects being thrown, in addition to verbal abuse," police said in a post on X.

Palestine Action's ban, or proscription, puts the group alongside al-Qaeda and Isis and makes it a crime to support or belong to the organisation, punishable by up to 14 years in prison.

Police had warned ahead of Saturday's demonstration that anyone showing support for the group would be arrested.

Human rights groups have criticised Britain's decision to ban the group as disproportionate and say it limits the freedom of expression of peaceful protesters.

The government has accused Palestine Action of causing millions of pounds worth of criminal damage and says the ban does not prevent other pro-Palestinian protests.

More than 100 people have been charged with showing support for the group in recent weeks. (Agencies)