The Supreme Court has now decided that the police have no power to arrest a person who is acting lawfully, even if the justification would be to prevent a potential crime from being committed.
For example, if you were involved in a protest, but you were acting lawfully, the police have no authority to arrest you based on a perceived apprehension that you may breach the peace.
The Supreme Court unanimously affirmed that any such power of arrest would substantially interfere with the liberty and freedom of law-abiding individuals and would not be reasonably necessary for the discharge of police duties.
The Court indicated they could not think of any basis for or circumstance in which arresting a person who is acting lawfully in order to prevent, in this case, a breach of the peace, would be reasonably justified.
In fact, according to the Supreme Court, “no such power exists”.
Fleming v. Ontario, 2019 SCC 45