Unionized Seoul Metro workers that operate subway lines one through eight launched a preliminary five-day strike on Friday.

The so-called work-to-rule campaign will last until next Tuesday, and entails by-the-book maintenance and mechanical checks that may delay the departure and arrival time of trains.

Seoul Metro​, which has warned of stern measures against workers who leave their work location without permission, said trains maintained normal operations during rush hour Friday morning. 

The labor union is demanding that management increase the number of workers and withdraw a plan to adopt a wage-peak system. If their demands are not met, the union plans to launch a three-day walkout next Wednesday. 

The union warned that it will stage a full-scale strike in mid-November if their demands are still not met.