An English court on Friday threw out a public order case against climate activist Greta Thunberg, the judge criticising "unlawful" conditions police had imposed on protesters.
District judge John Law dismissed the case against the 21-year-old Swedish campaigner and four other activists on the second day of their trial in London.
He ruled police had attempted to impose "unlawful" conditions during an environment protest in the British capital last October when they were arrested.
Thunberg, a global figure in the fight against climate change, was one of dozens of activists arrested for disrupting access to a major oil and gas conference attended by companies at a luxury hotel.
She had pleaded not guilty in November to breaching a public order law, alongside two protesters from the Fossil Free London (FFL) campaign group and two Greenpeace activists.
In his ruling, Law said the conditions imposed on protesters were "so unclear that it is unlawful", which meant "anyone failing to comply was actually committing no offence".
Thunberg, who came to worldwide attention as a 15-year-old by staging school strikes in her native Sweden, regularly takes part in such demonstrations.
She was fined in October for blocking the port of Malmo in Sweden, a few months after police forcibly removed her during a demonstration against the use of coal in Germany.
She also joined a march last weekend in southern England to protest against the expansion of Farnborough airport, which is mainly used by private jets.
Demonstrators had greeted the October forum participants with cries of "shame on you!". Some carried placards reading "Stop Rosebank", a reference to a controversial new North Sea oil field the British government authorised in September.
Police said officers had arrested Thunberg for failing to adhere to an order not to block the street where the rally was taking place.
Greenpeace UK campaigner Maja Darlington hailed Friday's verdict as "a victory for the right to protest".
She added: "It is ridiculous that more and more climate activists are finding themselves in court for peacefully exercising their right to protest, while fossil fuel giants like Shell are allowed to reap billions in profits from selling climate-wrecking fossil fuels."