The STM's Twitter feed reported the issues were caused by computer problems.
The service said it would increase service on all bus routes that run parallel to the four metro lines.
Hundreds of thousands of people in Montreal rely on the metro for transportation every day.
In 2012, Montreal's metro had more than 1,000 unplanned stoppages, making it the worst record for service delays in six years.
An internal report by the STM revealed that half of interruptions were caused by passengers.