LONDON (AP) — Thousands of London Underground staff on Sunday began a series of strikes over pay and conditions that threaten to shut down the subway system used by millions of people a day.

The Rail, Maritime and Transport Union said its members, including drivers, signalers and maintenance workers, will walk out at various times through Thursday.

Transit operator Transport for London said services were severely disrupted on Sunday and there are expected to be few or no trains running Monday through Thursday.

The company says it has offered staff a 3.4% pay increase, but the union is holding out for a reduction in the working week from 35 hours to 32 hours. TfL says it can’t afford that.

The union argues that the number of people working on the Underground has been cut by 2,000 since 2018, “and our members are feeling the strain of extreme shift patterns.”

No talks between union and management are currently planned, and the union called on Mayor Sadiq Khan to intervene to end the dispute.