Greta Thunberg has walked free from court after the district judge threw out the public order charge against her.
District Judge John Law said that the police had attempted to impose "unlawful" conditions during an environmental protest.
The 21-year-old was arrested after staging a demonstration outside a London hotel, where oil executives were meeting on October 17 last year.
The industry leaders were attending the Energy Intelligence Forum at the InterContinental Hotel in Mayfair.
Greta Thunberg celebrated the decision
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Thunberg pleaded not guilty to breaching Section 14 of the Public Order Act 1986 alongside two Fossil Free London (FFL) protesters and two Greenpeace activists.
They were accused of failing to leave the highway and continuing the protest on the pavement.
The Swede arrived at court yesterday all smiles, grinning at photographers before making her way inside the building.
Activists held up banners in support of Thunberg and the other defendants.
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Greta Thunberg grinned to the photographers outside court
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The large yellow signs read: "Climate protest is not a crime" and "Who are the real criminals?".
At Westminster Magistrates’ Court on Friday, District Judge John Law said conditions imposed on protesters were “so unclear that it is unlawful” which meant “anyone failing to comply were actually committing no offence”.
The judge said the protest was “throughout peaceful, civilised and non-violent” and criticised evidence provided by the prosecution about the location of where the demonstrators should be moved.
He said the only helpful footage he received was “made by an abseiling protester”.
He added: “It is quite striking to me that there were no witness statements taken from anyone in the hotel, approximately 1,000 people, or from anyone trying to get in.
“There was no evidence of any vehicles being impeded, no evidence of any interference with emergency services, or any risk to life.”
The court heard that protesters started to gather near the hotel in October last year at around 7.30am and police engaged with them about improving access for members of the public, which the prosecution alleged had been made “impossible”.
The judge rejected the submission as “the main entrance was accessible (meaning) that the condition… was unnecessary when the defendants were arrested”.
Speaking on behalf of Thunberg, lawyer Raj Chada said the charges made against her were "rightly dismissed" and that the Thunberg camp will "look into all options".