WARSAW—The Polish government said it would close four coal mines to stave off bankruptcy at Europe’s largest coal producer, a plan that was met with labor-union protests on Thursday.

Several hundred miners stayed underground after their shifts on Thursday to protest the plan that would see four mines belonging to Kompania Weglowa SA in the south of Poland closed down and 10 bailed out by state-controlled companies. Kompania Weglowa is owned by the government and isn’t publicly traded.

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