Greta Thunberg was arrested at a protest in October, with the judge ruling the law was unclear.
Greta Thunberg and four co-defendants have been found not guilty of breaking the law when they refused to follow police instructions to move on during a climate protest.
District Judge John Laws threw out a public order charge due to “no evidence” and added police attempted to impose “unlawful” conditions during a protest.
The 21-year-old campaigner was arrested at a climate change demonstration near the InterContinental Hotel in Mayfair on 17 October.
The judge said that the conditions imposed on protesters were “so unclear that it is unlawful”.
He added that it meant that “anyone failing to comply were actually committing no offence”.
“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,” he said.
“There was no evidence of any vehicles being impeded, no evidence of any interference with emergency services, or any risk to life.”
He said that 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 to, saying the only helpful footage he received was “made by an abseiling protester”.
The court heard that protesters started to gather near the hotel in October last year at around 07:30 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”.
Ms Thunberg appeared at Westminster Magistrates’ Court after previously denying breaching the Public Order Act 1986.
She is accused of breaching section 14 of the act by blocking the entrance to the hotel.
Ms Thunberg appeared at court along with two Fossil Free London protesters and two Greenpeace activists, who also pleaded not guilty to the same offence.
Oil executives had been meeting inside for the Energy Intelligence Forum.