Hundreds of Extinction Rebellion protesters may sue the Metropolitan police for unlawful arrest after the high court quashed an order banning the group’s protests in London last month.

Mr Justice Dingemans and Mr Justice Chamberlain said the Met’s section 14 order that XR “must now cease their protests within London” was unlawful because it went beyond the powers granted to police by the Public Order Act 1986.

In a judicial review ruling handed down on Wednesday morning, the judges said the Met had been wrong to define Extinction Rebellion’s two-week long “autumn uprising” as a single public assembly on which it could impose the order.

“Separate gatherings, separated both in time and by many miles, even if coordinated under the umbrella of one body, are not a public assembly under the meaning of section 14(1) of the 1986 act,” Dingemans said.

“The XR autumn uprising intended to be held from 14 to 19 October was not therefore a public assembly … therefore the decision to impose the condition was unlawful because there was no power to impose it.”

The ban was “hastily imposed [and] erratically applied”, said Jules Carey, partner at Bindmans and part of the team that brought the judicial review claim.

“This judgment is a timely reminder to those in authority facing a climate of dissent: the right to protest is a longstanding fundamental right in a democratic society that should be guarded and not prohibited by overzealous policing.”

Q&A What are Extinction Rebellion's key demands? Show Hide The UK group of Extinction Rebellion has three core demands: 1) Tell the truth

The government must tell the truth about the scale of the ecological crisis by declaring a climate emergency, “working with other groups and institutions to communicate the urgent need for change”. 2) Net zero emissions by 2025

The UK must drastically cut its greenhouse gas emissions, hitting net zero by 2025. 3) Citizens’ assembly

The government must create a citizens’ assembly to hear evidence and devise policy to tackle the climate crisis. Citizens’ assemblies bring together ordinary people to investigate, discuss and make recommendations on how to respond, in this case, to the ecological emergency. In the US activists have added a further demand: “A just transition that prioritises the most vulnerable and indigenous sovereignty [and] establishes reparations and remediation led by and for black people, indigenous people, people of colour and poor communities for years of environmental injustice.” Matthew Taylor

The strong judgment striking down the ban is a huge embarrassment for the Met, which had insisted in the face of criticism that it was lawful. It now faces potential claims from hundreds of protesters arrested for breaching the order, which ran across London from 9pm on Monday 14 October until 6pm the following Friday.

“It’s a very expensive mistake for the Metropolitan police,” said Carey. Anyone arrested under the order could now have a claim against the Met for false imprisonment, he said. If force was used against them, they could have a further claim for assault.

“I’m sure most of them would want to start off with an apology for the ordeal that they experienced, but all of them could potentially be awarded several thousands of pounds depending on how long they were arrested for and whether force was used against them.”

The judicial review was brought on behalf of XR by Jenny Jones, Caroline Lucas and Ellie Chowns of the Green party, the Labour MPs Clive Lewis and David Drew, the Labour activist Adam Allnutt and the Guardian columnist George Monbiot.

Monbiot, who was arrested while taking part in XR protests around Trafalgar Square directly addressing the Met’s imposition of the section 14 order, said he was considering taking legal action.

He said: “The important thing is that the attempt by the police to quash our democratic right to dissent has been overturned. Non-violent civil disobedience is essential to the health of our democracy, in fact, there would be no democracy without it. What the police were doing is draconian and over the top, and they were directly infringing our democratic right to protest.

“The Public Order Act itself is highly illiberal legislation which has a major chilling effect on protest, as do several of its successor acts … police have this wide range of tools for shutting down protest and one thing I would like to see coming out of this is much more discussion about those tools, the wide range of powers police have to shut down democracy.”

Play Video George Monbiot arrested for defying climate protest ban – video

Chowns, who was arrested in Trafalgar Square as she attempted to challenge police on their justification for the ban, is also taking legal advice on whether to sue for unlawful arrest.

The Met’s assistant commissioner, Nick Ephgrave, said the force was disappointed by the ruling and would consider its position, including whether to appeal. The decision to impose the ban “was not taken lightly” and followed “unacceptable and prolonged disruption to Londoners”, he said.

“After more than a week of serious disruption in London both to communities and across our partner agencies, and taking account of the enormous ongoing effort by officers from the Metropolitan police service and across the UK to police the protest, we firmly believed that the continuation of the situation was untenable,” Ephgrave said.

“I want to be clear: we would not and cannot ban protest. The condition at the centre of this ruling was specific to this particular protest, in the particular circumstances at the time.”

The civil rights groups Amnesty International, Liberty and Article 19 welcomed the judgment. “The sweeping, ill-defined and capital-wide ban sent the chilling message that basic freedoms in this country can be set aside when the authorities choose to do so,” said Kate Allen, director of Amnesty International UK.

“People are understandably deeply concerned at a lack of government action to tackle the climate crisis, and the authorities should be ensuring that those demanding climate justice are able to participate in non-violent protests.”

Kevin Blowe, coordinator of the Network for Police Monitoring, who was in court to see the judgment handed down, said it left police with serious questions to answer.

“Someone really needs to be held accountable for the decision,” he said. “What the police can’t do is simply say, ‘It was a difficult situation, we feel we acted proportionately’, and then move on as if nothing has happened.”

The police decision came despite misgivings from the London mayor, Sadiq Khan – who, along with the home secretary, is one of the two people to whom the force answers.

After the ruling, a spokesperson for Khan said he had been clear there was “a fine balance” between keeping London safe and protecting the right to protest.

“On this occasion, the operational decision taken independently by the Metropolitan police, about which Sadiq was extremely concerned at the time, was the wrong side of that line,” the spokesperson said.

“Following the high court ruling today, Sadiq will be seeking further assurances from the Met they will learn lessons from this to ensure future decisions are robust and that they will continue to uphold the right to peaceful and lawful protest.”

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The Met faces serious fallout from the judgment. Apart from the compensation claims and criticisms, public order commanders will have to work out which of their powers will survive the legal challenge.

With more protests expected in coming months, both by XR and over other issues such as Brexit, police have lobbied the government for law changes to make it easier to curtail disruptive protests.

One senior police source said changes could include lowering the threshold at which police can place restrictions. Such a change could mean that the prospect of “disruption” is enough to impose conditions, not “serious disruption” as the law currently requires.

Speaking before Wednesday’s judgment, the source said the government had told police it wanted to see more robust and proactive action – so-called “move forward” tactics – to clear the streets.