MARIUPOL, Ukraine — Ukrainian security forces in armored vehicles attacked a police station here on Friday, reducing the building to smoldering rubble and killing at least seven people, according to residents who say they witnessed the assault.

The attack signaled what appeared to be a major escalation in the interim government’s fight with pro-Russian militants in eastern Ukraine, and came two days after President Vladimir V. Putin of Russia urged that the separatists delay a referendum scheduled for Sunday and that all sides settle their differences through dialogue.

Hours after the assault, two bodies were still lying in the street, possibly victims of gunfire leveled by security forces against a crowd that broke away from a pro-Russian Victory Day march to lend support to mutinous police officers, residents said.

The death toll in Mariupol, a city of about 500,000, was expected to rise as rescue workers gained access to the police station, which exploded in flames. Hospital officials said they were treating 40 people for gunshot wounds and had sent five bodies to morgues, according to a Human Rights Watch researcher who had canvassed hospitals. The Kiev government said 20 rebels and one soldier had died.