Metropolitan Police Commissioner Sir Mark Rowley says he is “shocked and frustrated” at a planned protest in support of Palestine Action.
The demonstration, due to take place in Westminster later, comes as the government moves to ban the group under anti-terror laws.
Activists from Palestine Action hit the headlines last week after targeting RAF Brize Norton and damaging two military aircraft in a significant security breach.
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Home Secretary Yvette Cooper will update MPs on the move to proscribe the organisation, which would make it a criminal offence to belong to or support it.
Sir Mark said that – until this happens – the force has “no power in law” to prevent the protest from taking place, but lawbreakers will be “dealt with robustly”.
He added: “This is an organised extremist criminal group, whose proscription as terrorists is being actively considered.
“Members are alleged to have caused millions of pounds of criminal damage, assaulted a police officer with a sledgehammer and last week claimed responsibility for breaking into an airbase and damaging aircraft.
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“Multiple members of the group are awaiting trial accused of serious offences.”
He added that – while the right to protest is essential – “actions in support of such a group go beyond what most would see as a legitimate protest”.
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Over the weekend, Business Secretary Jonathan Reynolds said he could not rule out the possibility of a foreign power being behind Palestine Action.
Any move to proscribe the group must be debated and approved by MPs and peers.
Speaking to Sunday Morning With Trevor Phillips on Sky News, former justice secretary Lord Charlie Falconer suggested that vandalising aircraft would not solely provide legal justification for such a move.
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Lord Falconer said: “I am not aware of what Palestine Action has done beyond the painting of things on the planes in Brize Norton, they may have done other things I didn’t know.
“But generally, that sort of demonstration wouldn’t justify proscription so there must be something else that I don’t know about.”
Palestine Action has staged a series of protests in recent months – spraying the offices of Allianz Insurance in London, and vandalising Donald Trump’s golf course in Ayrshire.
