Climate activist Greta Thunberg was on Friday cleared of a public order offence as a judge ruled police had no power to arrest her and others at a protest in London last year.
Thunberg stood trial with four other defendants who were arrested on Oct 17 outside a London hotel, where the Energy Intelligence Forum was hosting oil and gas industry leaders.
All five were accused of failing to comply with an order made under the Public Order Act by police to move their protest to a designated area near the conference.
They were all acquitted at Westminster Magistrates' Court, in a ruling which throws into doubt other prosecutions of those facing the same charge from the Oct 17 demonstration.
Judge John Law ruled that London's Metropolitan Police acted unlawfully in imposing conditions on the protest and that therefore Thunberg had no case to answer.
He said that police could have imposed lesser restrictions on the protest and the conditions that were imposed were not clear.
Law also said Thunberg was not "given anything like a reasonable time to comply" after police told her to move.
Raj Chada, a lawyer who represented Thunberg and two other defendants, said outside court: "The government should stop prosecuting peaceful protesters and instead find ways to tackle the climate crisis."
Thunberg, who became a prominent campaigner worldwide after staging weekly protests in front of the Swedish parliament in 2018, made no comment to reporters as she left court.
A spokesperson for the Metropolitan Police said in a statement: "While we absolutely respect the right to protest, we often hear from Londoners who are fed up with repeated serious disruption at the hands of campaigners who block roads and prevent people going about their normal business.
"Officers have to balance these considerations in real time." They added: "We will review the decision carefully."
Prosecutors, who are likely to seek an adjournment of a similar trial starting next week, can bring an appeal at the High Court against Friday's decision.
Britain's Crown Prosecution Service did not immediately respond to a request for comment.