The safeguards are the bail court and, of course, the entire justice/trial system.
That said, to seek a warrant for arrest, yes, the police would have to present their case to a judge and convince them to endorse their application. Warrantless arrests can be done, however, under s. 495(1) of the Criminal Code:
- 495(1) A peace officer may arrest without warrant
- (a) a person who has committed an indictable offence or who, on reasonable grounds, he believes has committed or is about to commit an indictable offence;
- (b) a person whom he finds committing a criminal offence; or
- (c) a person in respect of whom he has reasonable grounds to believe that a warrant of arrest or committal, in any form set out in Part XXVIII in relation thereto, is in force within the territorial jurisdiction in which the person is found.
Challenging such warrantless arrests after the fact and getting evidence tossed on bad arrests is pretty much my bread-and-butter as a defence lawyer. Police make warrantless arrests at their own risk. Usually they're pretty careful to make sure they have the requisite grounds so that guys like me can't get important evidence excluded at trial.