At least 100 pro-Russian separatists attacked the police headquarters in eastern Ukraine's city of Horlivka on Monday.
Dozens of angry men hurled rocks, smashed windows and broke into a building, while hundreds of onlookers cheered them on. Thick white smoke rose from the entrance to the building, from which the insurgents hoisted the Russian flag.
Video footage on Ukrainian television showed an ambulance treating people who were apparently injured during the attack in Horlivka, which has a population of around 300,000 people.
"The police are with the people," men told a large crowd chanting their support, after walking out of the building.
The events in Horlivka were the latest sign of trouble in Russian-speaking eastern and southern regions, in which pro-Russian gunmen have seized or blocked government buildings in at least nine cities demanding more autonomy from the central government and closer ties with Russia.
Kiev authorities and Western officials have accused Moscow of instigating the protests, saying the events echoed those in Crimea, which was annexed by Russia last month.
Albina Kovalyova of NBC News, The Associated Press and Reuters contributed to this report.