MONTREAL—For the second time in three weeks, Montreal’s entire metro system came to a halt on Tuesday afternoon due to a computer glitch.

Service was interrupted on all four lines of the metro just after 6 p.m. The outage lasted about an hour.

A spokesperson for Montreal’s transit authority confirmed that shuttle buses were being dispatched to transport stranded passengers.

Metro service was interrupted at least twice in 2012 by similar computer-related problems. According to STM spokeswoman Amelie Regis, the more recent problems may have been caused by a switch to a new computer system.

Over 1 million people ride the metro every weekday.