Sir Mark Rowley said he was “shocked and frustrated” at a planned protest in support of Palestine Action, as the Government moves to ban the group.
The Metropolitan Police Commissioner said a protest supporting the “organised extremist criminal group” was due to take place in Westminster on Monday.
He said until the group is proscribed the force has “no power in law” to prevent the protest taking place, adding that breaches of the law would be “dealt with robustly”.
The Home Secretary will update Parliament on Monday on the Government’s plan to ban Palestine Action following the group’s vandalism of two planes at an RAF base.
Yvette Cooper will provide MPs with more details on the move to proscribe the group, making it a criminal offence to belong to or support it, in a written ministerial statement.
The decision comes after the group posted footage online showing two people inside the base at RAF Brize Norton in Oxfordshire.
The clip shows one person riding an electric scooter up to an Airbus Voyager air-to-air refuelling tanker and appearing to spray paint into its jet engine.
The incident is being investigated by counter-terror police.
In a statement on Sunday, Sir Mark said: “I’m sure many people will be as shocked and frustrated as I am to see a protest taking place tomorrow in support of Palestine Action.
“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. Multiple members of the group are awaiting trial accused of serious offences.
“The right to protest is essential and we will always defend it, but actions in support of such a group go beyond what most would see as legitimate protest.
“Thousands of people attend protests of a different character every week without clashing with the law or with the police. The criminal charges faced by Palestine Action members, in contrast, represent a form of extremism that I believe the overwhelming majority of the public rejects.
“We have laid out to Government the operational basis on which to consider proscribing this group. If that happens we will be determined to target those who continue to act in its name and those who show support for it.
“Until then we have no power in law to prevent tomorrow’s protest taking place. We do, however, have the power to impose conditions on it to prevent disorder, damage, and serious disruption to the community, including to Parliament, to elected representatives moving around Westminster and to ordinary Londoners.
“Breaches of the law will be dealt with robustly.”
A spokesperson for Palestine Action previously accused the UK of failing to meet its obligation to prevent or punish genocide.
The spokesperson said: “When our Government fails to uphold their moral and legal obligations, it is the responsibility of ordinary citizens to take direct action. The terrorists are the ones committing a genocide, not those who break the tools used to commit it.”
Cabinet minister Jonathan Reynolds said he could not rule out the possibility of a foreign power being behind Palestine Action.
People marching for Palestine are misdirecting their support
The Business and Trade Secretary told the BBC’s Sunday with Laura Kuenssberg: “It is extremely concerning they gained access to that base and the Defence Secretary is doing an immediate review of how that happened.
“The actions that they undertook at Brize Norton were also completely unacceptable and it’s not the first. It’s the fourth attack by that group on a key piece of UK defence infrastructure.”
The Home Secretary has the power to proscribe an organisation under the Terrorism Act of 2000 if she believes it is “concerned in terrorism”.
Proscription will require Ms Cooper to lay an order in Parliament, which must then be debated and approved by both MPs and peers.
Some 81 organisations have been proscribed under the 2000 Act, including Islamist terrorist groups such as Hamas and al Qaida, far-right groups such as National Action, and Russian private military company the Wagner Group.
Former justice secretary Lord Charlie Falconer said vandalising aircraft at RAF Brize Norton would not solely provide legal justification for proscribing the group.
Asked whether the group’s actions were commensurate with proscription, Lord Falconer told Sky News’s Sunday Morning With Trevor Phillips: “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.”
Former Scottish first minister Humza Yousaf said the Government was “abusing” anti-terror laws against pro-Palestine activists, as tens of thousands of protesters marched in London on Saturday.
Belonging to or expressing support for a proscribed organisation, along with a number of other actions, are criminal offences carrying a maximum sentence of 14 years in prison.
The Campaign Against Antisemitism (CAA) welcomed the news that Ms Cooper intended to proscribe Palestine Action, saying: “Nobody should be surprised that those who vandalised Jewish premises with impunity have now been emboldened to sabotage RAF jets.”
Former home secretary Suella Braverman also said it was “absolutely the correct decision”.
A pro-Palestine protester at Saturday’s march in central London said it was “absolutely horrendous” that the Government was preparing to ban Palestine Action.
Artist Hannah Woodhouse, 61, told the PA news agency: “Counter-terrorism measures, it seems, are being used against non-violent peace protesters.
“The peace activists are trying to do the Government’s job, which is to disarm Israel.”
Palestine Action has staged a series of demonstrations in recent months, including spraying the London offices of Allianz Insurance with red paint over its alleged links to Israeli defence company Elbit, and vandalising Donald Trump’s Turnberry golf course in South Ayrshire.
