Police in Odessa have freed some detained activists after hundreds of pro-Russian demonstrators stormed police headquarters in the Ukrainian port city.
The Interior Ministry said that 67 people who were detained after deadly clashes were released.
More than 40 people died in the clashes on Friday, some from gunshot wounds, but most in a fire that tore through a trade union building.
Ukrainian prime minister Arseniy Yatsenyuk visited Odessa to try to defuse mounting tensions. He said police are being investigated for their failure to maintain order and hinted strongly that he sees Moscow's hand in the unrest.
Odessa is the major city between the Crimean Peninsula, which Russia annexed in March, and the Moldovan separatist region of Trans-Dniester, where Russia has a military peacekeeping contingent.
Concerns are mounting that Russia ultimately aims to take control of a huge swath of Ukraine from Trans-Dniester to the east. Russian president Vladimir Putin, who calls the area historically Russian lands, has said he does not want to send in troops but will if necessary.
Mr Yatsenyuk said police were being investigated for their failure to maintain order and he had charged prosecutors with "finding all instigators, all organisers and all those that under Russian leadership began a deadly attack on Ukraine and Odessa".
Earlier in the day, hundreds of pro-Russian demonstrators gathered in front of the scorched trade union building to honour those who died in Friday's blaze. Some draped a large Russian tricolour flag on the face of the building.
By mid-afternoon, a group of several hundred people marched to the police station to demand the release of fellow activists jailed over their involvement in the unrest. They attacked security surveillance cameras and smashed windows. Shortly after some of them managed to break into an inner courtyard, police yielded to the crowd's demands and released the prisoners.
As detainees emerged from the police station, the crowd cheered.
The Interior Ministry said in a statement that 67 activists had been released. It was not immediately clear whether others were still being held.
Mr Yatsenyuk's visit came as Ukrainian authorities renewed their push to quell a pro-Russian insurgency in the east. Interior minister Arsen Avakov said in a statement on his Facebook page that an "anti-terrorist operation" was being executed in the eastern city of Kramatorsk, the latest flashpoint for unrest.
"The operation was carried out by fighters of the National Guard and the armed forces. The active phase resumed at dawn. We will not stop," Mr Avakov wrote.
The city saw a stand-off yesterday that culminated in pro-Russian insurgents setting buses alight to ward off attacks. Russian state television has reported 10 deaths, including two among government forces, during clashes in Kramatorsk so far. Those figures could not be independently confirmed.
By midday today, however, there was little sign of movement, from either government or insurgents on the ground. The burned-out shells of trolleybuses and a minibus lay in the road untouched.