Montreal police are investigating after bags of bricks were thrown onto the tracks and emergency brakes were pulled in several metro stations this morning.

The disruption caused service to be halted on three metro lines at the start of rush hour. Trains started up again about 30 minutes later.

Montreal transit corporation spokeswoman Isabelle Tremblay said as many as 15 people were involved in pulling the emergency brakes around 7:15 a.m. ET.

"When that emergency brake is pulled, we have to stop to check what's going on," she said.  "It was manually done. So we can say that it was a concerted action."

She said police are investigating to determine who was behind the co-ordinated disruption.