Donetsk - A crowd of some 300 pro-Russian militants attacked the prosecutor's office in the eastern Ukrainian city of Donetsk on Thursday, AFP reporters on the scene said.
The mob hurled rocks and Molotov cocktails at around 100 riot police defending the building, who responded with stun grenades and tear gas, in the latest unrest to hit the crisis-wracked eastern part of Ukraine.
The crowd stripped weapons and shields from some of the police, several of whom fled.
At least four police officers were wounded in the clashes as the crowd chanted "fascists, fascists" as it set upon them.
The building came under attack from several sides, according to AFP reporters. There were no visible signs of firearms being used.
The clash came as some 10 000 pro-Russian supporters staged a demonstration in the city that had begun peacefully.
Donetsk, an industrial hub of one million people, is the heart of what separatists call their "Donetsk Republic".
It was already the scene of violent clashes on 28 April when men armed with knives, baseball bats and iron bars attacked a pro-Kiev rally.
Rebels seized the regional administrative building on 6 April and the city hall on 16 April.
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.
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 11 May on independence from Kiev and closer ties with Russia.