Open this photo in gallery The Supreme Court of Canada says no police action should ever be justified solely by the fact it meets a desired end. Sean Kilpatrick/The Canadian Press

Imagine you’re planning to attend a protest to express your views on a contentious issue and that at the rally, there will be people whose opinions are opposed to yours.

On your way to the event, with your colourfully worded picket sign on your shoulder, a police officer approaches you and asks where you’re going. You tell him. He arrests you.

You are understandably dismayed. You ask why you’re being detained. The officer explains that – even though you’ve done nothing wrong – he is concerned that your act of protest might provoke a counterprotester to assault you. You are held in jail for three hours before you are released. You’re also charged with resisting arrest.

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That precise scenario occurred in Caledonia, Ont, a decade ago. Last week, the Supreme Court of Canada confirmed just how wrong and absurd it was.

In 2009, Randy Fleming was walking to a “flag rally” – one of a series of protests during a long standoff between Caledonia residents and a group of First Nations protesters occupying a disputed parcel of land. The standoff had flared into violence on numerous occasions.

On his way to the join the rally, carrying a Canadian flag on a pole and heading in the direction of the entrance to the disputed land, Mr. Fleming was passed by three Ontario Provincial Police vehicles going in the opposite direction. The police were there to keep the opposing sides apart.

On seeing Mr. Fleming, the police turned their vehicles around, but in doing so, they forced him off the shoulder. Mr. Fleming crossed a ditch and climbed over a small fence onto the disputed land.

Several officers yelled at him to return to the shoulder. Off in the distance, about 100 metres away, a group of First Nations protesters started to walk or jog toward Mr. Fleming. He remained where he was.

As the protestors approached, a police officer told Mr. Fleming he was under arrest and marched him off the disputed land. The officers then ordered him to drop his flag. He refused. The officers forced him to the ground and, in the process of handcuffing him, permanently injured his left arm.

Mr. Fleming was held for 2 1/2 hours and charged with resisting arrest. He appeared in court 12 times before the charge was dropped 19 months later.

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He subsequently filed a suit against the police and the provincial government. He won in a lower court, lost on appeal and was vindicated by last week’s Supreme Court decision. His efforts have resulted in a ruling that clearly defines the proper limits of police powers in Canada.

The OPP had no statutory grounds for arresting Mr. Fleming. He hadn’t committed an offence and was not suspected of being about to do so. But police argued that a common-law precedent allows them to apprehend a law-abiding person in order to prevent an anticipated breach of the peace. The police were not anticipating that Mr. Fleming was about to break the law; they rather worried that people who disagreed with his protest were about to break the law by attacking him.

Given the sometimes violent nature of the lengthy Caledonia standoff, two judges on the Ontario Court of Appeal agreed that the police were justified in arresting Mr. Fleming. The OPP officers knew that minor incidents could turn into serious clashes, and the appeals court ruled that the police had taken legal actions to keep the peace.

The Supreme Court found otherwise. Even if the arrest of Mr. Fleming had indeed prevented some sort of violence from occurring – the facts of the case don’t bear out that claim – it wasn’t justified.

In fact, the court said, no police action should ever be justified solely by the fact it meets a desired end. “That is a recipe for a police state, not a free and democratic society,” the ruling correctly concludes.

Detaining a person is the most extreme infringement on personal liberty allowed in a free society; it becomes even more fraught when a police officer arrests a hypothetical future victim of a breach of the peace who is merely expressing an opinion. Canadians must be free to express opinions in public in a peaceful manner – a right that can’t be pre-empted by the worry that others might respond with violence.

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Otherwise, it would be too easy to shut down protests before they even start. Mr. Fleming’s 10-year ordeal has made Canadians’ freedom of expression a little more secure.