The short answer is no. Police can’t arrest someone who isn’t breaking the law to prevent others from breaching the peace, the Supreme Court has ruled. The Supreme Court, in Fleming v. Ontario 2019 SCC 45, unanimously said the officers didn’t have the power to arrest Mr. Fleming. This decision is somewhat timely with the recent arrest of dozens of protesters in Halifax and Toronto, and fist fights reportedly breaking out in Edmonton, as climate change activists blockaded several major commuter bridges in Canadian cities: https://www.cbc.ca/news/canada/nova-scotia/macdonald-bridge-halifax-climate-change-1.5310569
Mr. Fleming was on his way to join a protest in Caledonia, Ontario in 2009. The protest was against the occupation of a piece of land by a First Nations group. The police became aware of the flag rally in the months preceding it and had developed an operational plan, given the contentious atmosphere in the community which had on numerous occasions culminated in violent clashes between the two sides. The plan included keeping protestors and counter‑protestors apart, and flag rally counter‑protestors were informed that they were not allowed on the occupied property. Mr. Fleming was carrying a Canadian flag on a wooden pole and walking down a street beside the occupied land.
Police officers saw him as they drove by. The officers turned their vehicles around and sped toward him. Mr. Fleming got off the road and crossed a low fence. He said he did this to get away from the speeding vehicles and onto level ground. The officers were yelling. Mr. Fleming said he didn’t think they were yelling at him because he hadn’t done anything wrong.
The people occupying the land came toward him. When they were about ten or twenty feet away, the police told Mr. Fleming he was under arrest. They ordered him to drop his flag. He refused. He was forced to the ground, handcuffed, placed in a transport unit van, moved to a jail cell and released two and a half hours later. He was charged with obstructing a police officer. He went to court a dozen times to fight the charge, which was later dropped. Fleming said police injured his arm.
In 2011, Mr. Fleming sued the Province of Ontario and the officers involved in his arrest. He said the officers acted wrongfully. He said they assaulted and battered him, wrongfully arrested him, and falsely imprisoned him. He also said they violated several of his rights under the Canadian Charter of Rights and Freedoms. Fleming was successful at trial, but a majority of the Court of Appeal set aside the award of damages on the basis that the police had the authority at common law to arrest him. The Court of Appeal ordered a new trial solely on the issue of excessive force. Fleming appealed to the SCC on the issue of whether the police acted lawfully in arresting him, and on whether a new trial should have been ordered on the question of excessive force.
The appeal centered around the purported police power to arrest someone who is acting lawfully in order to prevent an apprehended breach of the peace by others. It targeted individuals who are not suspected of being about to break any law or to initiate any violence themselves, in situations in which the police nonetheless believe that arresting the individuals in question will prevent a breach of the peace from occurring. To reiterate, the arrest of Fleming did not concern a power to arrest a person for the purpose of preventing that person from breaching the peace, but instead arresting him for lawful conduct which may provoke others to breach the peace.
At common law, arguably, authority exists that a police officer is entitled to make a lawful arrest of someone “who it is anticipated may shortly engage” in a breach of the peace (see, for example, R. v. Khatchadorian (1998) 127 C.C.C. (3d) 565 (B.C.C.A.), R. v. Lefebvre (1984) 15 C.C.C. (3d) 503 (B.C.C.A.), Hayes v. Thompson (1985) 18 C.C.C. (3d) 254 (B.C.C.A.), R. v. Faulkner (1988) 9 M.V.R. (2d) 137 (B.C.C.A.), Brown v. Durham (Regional Municipality) Police Force (1998) 43 O.R. (3d) 223 (Ont.C.A.) appeal to S.C.C. granted  S.C.C.A. No. 87), or if a breach of the peace is reasonably expected to occur. This authority is like the warrantless power of arrest under s. 495(1)(a) of the Criminal Code for a person “about to commit an indictable offence”. The arrest for an apprehended breach of the peace is exercised in circumstances where the officer has reasonable grounds for believing the anticipated conduct, which would amount to a breach of the peace, will likely occur if the person is not arrested. Unlike section 31 of the Code, which requires the existence of an actual breach of the peace before the police may arrest, police officers need not be concerned themselves with what has occurred, but with what is reasonably expected to occur. There are two requirements that the police officer must consider when exercising this power: the apprehended breach must be imminent. The possibility that a breach will occur at some unknown point in time will not be sufficient. The breach must be impending and likely to occur in the immediate future; and, the apprehended breach must be substantial. The possibility of an unspecified breach will also be insufficient. The likelihood of a particularized and identifiable breach must be real and reasonably apprehended. There must be articulable indications that the conduct that forms the nature of the breach will occur (see, for example, the cases of Brown v. Durham (Regional Municipality) Police Force (1998) 43 O.R. (3d) 223 (Ont.C.A.) appeal to S.C.C. granted  S.C.C.A. No. 87 and Lynch v. Canada (R.C.M.P.) 2000 BCSC53). But, again, in this case, the arrest of Fleming did not concern a power to arrest a person for the purpose of preventing that person from breaching the peace, but instead arresting him for lawful conduct which may provoke others to breach the peace.
The SCC ruled that the police can’t arrest someone acting lawfully just because they think it will stop others from breaching the peace. Police already have other powers to deal with these situations under the Criminal Code. Since police had these less drastic options, arresting Mr. Fleming wasn’t really necessary. The Court noted that preserving the peace, preventing crime, and protecting life and property are the main duties of police officers under the common law. Police have the power to take actions to support these duties, even if these actions aren’t specifically set out in the Criminal Code. Preventing breaches of the peace is obviously related to preserving the peace, preventing crime, and protecting life and property. But the Court said it wasn’t reasonably necessary to arrest someone to prevent a breach of the peace, if that person hadn’t done (and wasn’t about to do) anything wrong.
Police are allowed to use as much force as reasonably necessary to carry out their duties. But in this case, they weren’t allowed to arrest Mr. Fleming, so no amount of force was justified. The Court allowed the appeal, set aside the order of the Ontario Court of Appeal and restored the trial judge’s order. Costs were awarded throughout: costs in this Court and the agreed-upon trial and appeal costs of $151,000 and $48,000 respectively.
Of note, in obiter of this decision, Justice Côté for an unanimous court said while it is not necessary to decide whether “a police officer may also arrest or detain a person who is about to commit a breach of the peace” in this case at common law, she seriously questions whether a common law power of this nature would still be necessary in Canada today. The Criminal Code provides explicitly for a number of warrantless arrest powers that obviate the need for such a common law power (e.g. ss. 31(1) and 495(1)(a) CC). Thus, police officers already have extensive powers to arrest, without a warrant, a person they reasonably believe is about to commit an act which would amount to a breach of the peace. She therefore had difficulty seeing any need for the courts to fill a legislative gap by recognizing a common law power of arrest for the purpose of preventing individuals from committing breaches of the peace themselves, and she made no comment about other possible powers short of arrest in such circumstances.