The STM is warning there could be delays Friday morning as tensions rise between the public transit authority and its maintenance workers.

In what is already expected to be a problematic morning due to Montreal’s first snowfall of the season, there will also be delays on buses and metros.

Some 500 mechanics walked off the job Thursday afternoon in what the STM is calling an “illegal” two-hour strike.

“It is, for us, unacceptable to take hostage of our clientele like that,” said the STM's general manager Luc Tremblay in a late-afternoon news conference.

The maintenance workers have been in contract talks for more than 18 months.

Tremblay couldn't specify just what effect it might have on Friday's rush hour or how many buses might be held back.

Tremblay was defending the STM against some of the allegations workers have been making in the media this week, blaming poor management for a shortage of buses on the road or for buses breaking down a lot more than they do in other major cities.

Tremblay said he was surprised to see the walk-out given that both parties were negotiating Wednesday night.

“We were negotiating with our union, and out of the blue they go on an illegal strike,” said Tremblay.

Negotiations have been difficult, with working hours at the heart of the conflict.

The STM said it wants to create more evening and overnight shifts, but the union said it is concerned about the workers' quality of life.

The dispute has resulted in thousands of hours of bus service not being provided because of overtime strikes.

Tremblay said a meeting with the labour tribunal is scheduled for Thursday night seeking an order to ensure a walk-out doesn't happen again in the coming days – especially with snow on the way.