District Judge John Law said on Friday that police had acted unlawfully during an environmental protest in London last year, when they arrested her for refusing to move.

Ms Thunberg was arrested outside the InterContinental Hotel in Mayfair, central London, where a major annual conference of oil executives was taking place, including representatives from Shell, Total, Equinor and Saudi Aramco, on 17 October last year.

She was handcuffed before being taken to a police van.

The 21-year-old Swede, along with two members of Fossil Free London (FFL) and two from Greenpeace, pleaded not guilty to breaching Section 14 of the Public Order Act 1986.

At Westminster Magistrates' Court on Friday, Judge 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 protest, he said, was "throughout peaceful, civilised and non-violent" and he criticised prosecution evidence about the location of where the demonstrators should be moved to - saying the only helpful footage he received was "made by an abseiling protester".

He said he found it "striking" that there were no witness statements taken from anyone in the hotel, approximately 1,000 people, or from anyone trying to get in.

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"There was no evidence of any vehicles being impeded, no evidence of any interference with emergency services, or any risk to life."

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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".

But the judge rejected the police view, as "the main entrance was accessible (meaning) that the condition... was unnecessary when the defendants were arrested".

It was not the first time Ms Thunberg had been taken away by police at a demonstration.

In July last year, she was forcibly removed from a protest in southern Sweden hours after a local court fined her for disobeying police during a similar demo a month earlier.

The campaigner admitted she had disobeyed the police order but pleaded not guilty and said she was acting out of necessity.