Pro-Russian activists have forced the release of several people held over deadly violence two days ago, after they attacked the police headquarters in Ukraine's southern city of Odessa.
Friday's clashes led to more than 40 deaths. Most of the victims were pro-Russian separatists killed when the building they were in caught fire.
Ukraine's interim Prime Minister has accused police of failing to prevent the violence.
He was speaking as Ukrainian troops encircled the city of Sloviansk in the east.
The army operation is trying to wrest control from separatists who have taken over government buildings in a number of towns.
Hundreds of people descended on the police headquarters in Odessa on Sunday afternoon in an initially peaceful protest calling for the release of those detained.
"We've gathered here in a protest against the illegal government and to get our guys released, citizens of Odessa", said one woman.
"They call us separatists and pro-Russians, but we're just ordinary Russian Odessa citizens."
The gathering turned violent as protesters - some wearing masks and carrying improvised weapons - broke windows and forced the gates.
A number of detainees were then released by the police, apparently in an attempt to pacify the crowds. The interior ministry, in a statement quoted by the Associated Press, said 67 people were released.
Moscow and Kiev have blamed each other's supporters for the fire.
Millions of ex-president's money frozen in Switzerland
A Swiss newspaper says Switzerland's federal prosecutor has frozen more than $NZ220 million of assets in Swiss bank accounts belonging to the former Ukrainian president Viktor Yanukovich and those close to him, including his son.
The newspaper says the federal prosecutor has opened five investigations against individuals suspected of money-laundering.
Moscow-friendly Mr Yanukovich was ousted in February following mass public protests.