DONETSK, Ukraine—The woman had just delivered homemade sandwiches to her son, a police officer working at the main prosecutor's office, when he called her with troubling news: He'd gotten hit with a rock on the head and wasn't feeling well.

The connection was lost before she could figure out where he was. So she rushed back and began searching floor by floor—alongside an unruly crowd of pro-Russian militants who had just seized the building.

"If they find him first, they'll kill him," the worried mother said as she hurried down slippery stairs strewed with broken glass. "Maybe he's in the basement. Where's the basement?"

The clash in this regional capital left a total 26 people injured, two of them requiring hospitalization, according to the Interior Ministry. It also left the pro-Russian protesters in control of another symbol of authority in the restive region, which is planning to hold a referendum on pulling away from Ukraine in less than two weeks.

Russian President Vladimir Putin on Thursday called on Ukraine to withdraw its military forces from the southeast of the country—a move that would effectively cede control to the pro-Russian forces that have taken over about a dozen cities in the border area.