Kiev: A crowd of some 300 pro-Russian militants hurling rocks and Molotov cocktails seized control of the prosecutor's office in the eastern Ukrainian city of Donetsk on Thursday.
Several of the 100 police officers guarding the building were injured and in some cases severely beaten as the mob stripped them of their weapons and shields, AFP reporters on the scene said.
Rebels quickly took control of the building after mounting a second assault from the back, hoisting the Russian flag, barricading the front door and burning Ukrainian symbols in the street.
The outnumbered police injured some rebels as they responded with stun grenades, tear gas and rubber bullets, but were quickly overpowered.
There was no visible sign of live ammunition being used by either side.
Once inside the building, which was empty for the May 1 public holiday, dozens of insurgents, dressed in civilian clothes and wearing balaclavas, tore down official photos and began a search for weapons.
Many of the captured police officers were abused and slapped as they were led through an angry crowd but then eventually released, if uninjured, or allowed to climb into waiting ambulances if hurt.
The clash, the latest unrest in the increasingly lawless east of Ukraine, erupted after what started as a peaceful demonstration by some 10,000 pro-Russian supporters in Donetsk.
An industrial hub of one million people, the city is the heart of what separatists call their "Donetsk Republic".
It was already the scene of violent clashes on April 28 when men armed with knives, baseball bats and iron bars attacked a pro-Kiev rally.
Rebels seized the regional administrative building on April 6 and the city hall on April 16.
The Western-backed government in Kiev has admitted it has effectively lost control of the increasingly
chaotic east, with interim president Oleksandr Turchynov saying authorities were "helpless" to prevent rebels over-running towns.
Turchynov has also criticised the police force for "inaction" and "in some cases treachery" as law enforcement bodies have stood aside while pro-Kremlin separatists seize buildings.
More than a dozen towns and cities in eastern Ukraine are already in the grip of the separatists, who have vowed to hold a referendum on May 11 on independence from Kiev and closer ties with Russia.