The move took effect Monday as negotiators for the Amalgamated Transit Union and the city continue contract negotiations.
The city says the action means some buses may not show up on schedule, and service on some routes may be cancelled.
John Callahan, a union spokesman, says up to 30 runs per day are done by drivers on overtime.
Handi-Transit customers are not affected, as that service is handled by private contractors.
Negotiations were scheduled to resume Monday after the union rejected an offer last Friday.
Callahan says the main issues are wages and the need for more staff.
"We don't want to inconvenience the public anymore than they are going to be inconvenienced by this. Our goal is to prove a point that you can't run a system this lean that you're dependent on overtime in order to get the service out," he said.
Transit riders are being asked to visit the Winnipeg Transit website, or call 311 for information about delays or cancellations.
The last time there was a transit strike in Winnipeg was 1976. (CJOB)