Police cannot arrest lawful protesters to prevent violence by others, the Supreme Court ruled on Friday.

The decision comes in the case of Randy Fleming, who carried a Canadian flag on a pole as part of a counterprotest against an Indigenous occupation of Crown land in Caledonia, Ont., in 2009.

It has wide implications for peaceful protests in Canada. The police, the court said, have no authority under the English common law (a centuries-old body of precedents) to arrest provocative but lawful protesters – even when police fear others may engage in violence.

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It was the first Supreme Court ruling in Canadian history to address police powers in such circumstances, the court said. Two lower courts that dealt with the case both accepted that such an arrest power existed.

“This is the authority that police used to justify the biggest mass arrests in Canadian history in the G-20 debacle,” lawyer Sean Dewart said, referring to the preventive arrests of more than 1,000 people in 2010 in downtown Toronto. Mr. Dewart represented the Canadian Civil Liberties Association, which intervened in the Fleming case. “Democracy and freedom of expression are considerably more safe than they were as a result of this ruling.”

Mr. Fleming had broken no law with his flag protest, police told the Supreme Court. But when eight to 10 people from an Indigenous demonstration headed toward him, Ontario Provincial Police officers ordered Mr. Fleming to drop the flag. He refused, and five officers pulled his arm behind his back, forced him to the ground and handcuffed him. He accused the police of an unlawful arrest, and causing him lasting physical injuries.

A charge of resisting arrest was dropped 19 months later, after he had made 12 court appearances.

Mr. Fleming sued police in 2011. An Ontario Superior Court judge ordered the OPP to pay him $139,711 in damages – including $5,000 for violation of his constitutional right to free speech – and $151,000 in legal costs. But the OPP appealed, and won 2-1 at the Ontario Court of Appeal, which threw out the damages award, saying the police tactic was effective in preventing violence.

The Supreme Court restored the damages order (adding appeal costs of $48,000), and sharply criticized the appeal court for stressing the effectiveness of the police action when constitutional rights were at stake.

“That is a recipe for a police state, not a free and democratic society,” Justice Suzanne Côté wrote in a 7-0 ruling. “It would directly undermine the expectation of all individuals, in the lawful exercise of their liberty, to live their lives free from coercive interference by the state.”

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She stressed that in general, arrests of peaceful protesters when no charges are laid pose a danger to civil liberties because no court would be involved, and therefore no judge would review police actions.

“The purported power in this case would directly impact on a constellation of rights that are fundamental to individual freedom in our society,” she wrote.

Mr. Fleming declined to be interviewed, but one of his lawyers, Michael Bordin, said his client feels his victory is an important day for the rule of law. “This has been a long, lonely fight, in which he faced all the resources of the government. He has been waiting for this vindication since his arrest on May 24, 2009. More than anything else, he is relieved. A burden has been lifted from his shoulders, a burden that he has often felt he was carrying alone, for all law-abiding Canadians.”

Police have arrest powers under the Canadian Criminal Code for such things as resisting or obstructing officers carrying out their duty, but only when they use less intrusive methods in circumstances such as Mr. Fleming’s case, the court said. The OPP did not argue that any Criminal Code powers applied to the situation involving Mr. Fleming.

A spokesperson for the OPP said the force was not ready to comment on Friday. Lawyer Bryant Mackey, representing the Canadian Association of Chiefs of Police, which intervened in the case, said the court’s recognition of the powers of arrest under the Criminal Code is “helpful, and undoubtedly will continue to be used in appropriate circumstances to preserve the peace during public protests.”

The incident arose from an at-times violent dispute in which protesters from the Six Nations of the Grand River reserve 25 kilometres south of Hamilton, Ont., occupied a housing development in nearby Caledonia in 2006. A court ordered them to leave. After the Crown bought the land and permitted the protesters to remain, Mr. Fleming arrived in counterprotest on May 24, 2009.