Six activists detained by police while protesting against tree felling in Sheffield were arrested on false grounds, the police watchdog has found.

The four men and two women were arrested between November 2016 and February 2017 “for the prevention of harm and injury” under an obscure clause of the Trade Union and Labour Relations Act.

The law was designed to ensure the lawful operation of trade unions yet was used against the Sheffield protesters to prevent them getting in the way of the chainsaws.

Those arrested always argued the law was being abused and now the Independent Office for Police Conduct (IOPC) has upheld a complaint by the protesters that their arrest and detention was inappropriate.

Calvin Payne, Margaret Mark, Kate Billington, Jeremy Peace, Simon Crump and Paul Brooke were arrested while trying to stop trees being cut down in Sheffield by Amey, a private contractor tasked by the council to fell 17,500 mature street trees and replace them with saplings.

Crump, an author and university lecturer, said: “These were punitive arrests carried out just to get us off the streets so that they could let Amey carry on with their environmental vandalism. There was never any realistic chance of a prosecution. They knew we would never end up in court – they just wanted us out of the way.”

He and the others were seeking damages from South Yorkshire police (SYP) for wrongful arrest, he said. “I was locked up for nine hours. I’m not sure how much I should get for that, but I think there’s a scale. I want as much money as possible, even though I would prefer an apology.”

A sign on a tree cut down by contractors in Rustlings Road, Sheffield.
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 A sign on a tree cut down by contractors in Rustlings Road, Sheffield. Photograph: Danny Lawson/PA

An IOPC spokeswoman said: “South Yorkshire police investigated complaints about arrests they made during a number of incidents that took place in Sheffield, between November 2016 and February 2017. Following the police’s investigation, we received six appeals from complainants who were not satisfied with the force’s investigation findings.

“After reviewing the police’s investigation, we upheld the complaints in relation to the grounds on which the arrests were made. Though we found there was reasonable suspicion that an offence had been committed, the protesters were arrested for the prevention of harm and injury; there were no grounds for this as there was no risk of injury. We have recommended management action for the three officers who made the arrests.”

South Yorkshire police said: “It is important to understand the IOPC did not question the officers’ motivation for the arrests, but questioned whether they were appropriate. No officers were found to have a case to answer for misconduct.

“These findings underline the complexity our officers face in dealing with competing interests, where they often have only a split second to make decisions, which are then examined at length for weeks or months, not just in terms of their legality but also around wider issues of context.

“We will be discussing with the IOPC their interpretation and relevance of the term ‘appropriate’ as it is not one that features in the relevant legislation or case law.”

Last year, Michael Gove, the environment secretary, called the felling programme “bonkers” and said the government would examine “legal or policy avenues” to end the scheme that had led to two years of protests by residents.

Sheffield city council paused the felling in March because of the “actions of a handful of people unlawfully entering the safety zones where tree replacement work is being carried out”. Work has not resumed.