There were no grounds to arrest six protesters for the prevention of harm and injury in a row about tree-felling in Sheffield, the police watchdog said.
All six were held between November 2016 and February 2017 under the Trade Union and Labour Relations Act.
They were charged but the criminal cases were discontinued following a Crown Prosecution Service review.
The Independent Office for Police Conduct (IOPC) upheld complaints about the grounds on which arrests were made.
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