The police, in carrying out their general duties as law enforcement officers of the state, have limited powers and are only entitled to interfere with the liberty or property of the citizen to the extent authorized by law. Absent explicit or implied statutory authority, the police must be able to find authority for their actions at common law. Otherwise they act unlawfully.

To determine whether a particular police action that interferes with individual liberty is authorized at common law, the ancillary powers doctrine must be applied. Fundamental to this doctrine is whether the police action is reasonably necessary in order to fulfil a statutory or common law duty of police (for instance, preserving the peace, preventing crime and protecting life and property).

In Brown v. Durham Regional Police Force (1998), 43 O.R. (3d) 223, the Court of Appeal for Ontario accepted (in obiter) that the police have a common law power of arrest to prevent an apprehended breach of the peace, provided that the apprehended breach is imminent and the risk of it occurring is substantial.

Writing for the Supreme Court of Canada in Fleming v. Ontario, 2019 SCC 45, at para. 60, Justice Côté (also writing in obiter) noted, “While it is not necessary to decide this in the instant case, I seriously question whether a common law power of this nature would still be necessary in Canada today.”

However, Justice Côté  was clear that there is no common law power to arrest someone who is acting lawfully to prevent an apprehended breach of peace by other persons. 

Lawful conduct does not become criminal because a natural and probable result of that conduct will be to provoke others to violent retributive action.
While the presence of a particular  individual at a particular place might be the catalyst for a breach of peace, and the arrest of such a person may advance the police duty of preserving the peace, as well as protecting life,  such an arrest would result in serious interference with individual liberty and is not reasonably necessary in light of existing the statutory powers of arrest available to police officers in such situations.
If the police can reasonably attain the same result by taking an action that intrudes less on liberty, a more intrusive measure will not be reasonably necessary no matter how effective it may be. An intrusion upon liberty should be a measure of last resort, not a first option.

Stuart O’Connell, O’Connell Law Group (All rights reserved to author)



This is because the Criminal Code provides explicitly for a number of warrantless arrest powers that obviate the need for such a common law power: s. 31(1), a police officer can arrest anyone found committing a breach of the peace or who the officer believes is “about to join in or renew the breach of the peace”; section 495(1)(a) provides that an officer can arrest any person “who, on reasonable grounds, he believes has committed or is about to commit an indictable offence”.
Frey v. Fedoruk, [1950] S.C.R. 517, at p. 526.
For instance, under section 129 of the Criminal Code, it is an offence to omit, without reasonable excuse, to assist a police officer in the execution of his duty in preserving the peace after having reasonable notice that you are required to do so.  As the 129 offence may be prosecuted by indictment, a person committing the offence may be arrested without a warrant under s. 495(1).
Fleming v. Ontario, at paras. 54, 98: The concept of reasonable necessity requires that other, less intrusive, measures not be valid options in the circumstances. If the police can fulfill their duty by an action that interferes less with liberty, the purported power is clearly not reasonably necessary.