Until this week’s violence, Odessa had been relatively calm compared to the more fractious eastern part of the country seen as a stronger bastion for pro-Russian Ukrainians.

Earlier in the day, hundreds of pro-Russian demonstrators gathered in front of the scorched trade union building to honor those who died in Friday's blaze. Some draped a large Russian tricolor 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.

Yatsenyuk's visit to Odessa 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 "antiterrorist 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," Avakov wrote.

The city saw a standoff Saturday 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.

Opposing sides of the Ukraine conflict have traded bitter recriminations over the mass deaths that followed hours of bloody rioting on Friday in Odessa.

The clash began with street fighting in which at least three people were reported killed by gunfire, then turned into a grisly conflagration when government opponents took refuge in a trade union building that caught fire as opposing sides hurled Molotov cocktails at one another.

The city's police chief, Petr Lutsyuk, on Saturday issued a statement calling for calm in the city of about 1 million, but hours later he was fired by the Interior Minister.

The fate of those killed in the trade union building has already become a rallying cause for resistance to the authorities by pro-Russians in the east. In a position eagerly promoted by the Kremlin, critics of the government have blamed those deaths on radical ultranationalists abetted by the government.

Pro-unity activists have argued, meanwhile, that their rally came under assault, including from attackers bearing firearms — leading to the deadly blaze.

Efforts to counteract the insurgency have focused mostly on the nearby town of Slovyansk; authorities are currently seeking to form a security cordon around that city.

It is difficult to establish what degree of popular support the gunmen in effective control of Slovyansk truly enjoy. The insurgency has proven hostile to supporters of the interim government that came to power in February after the toppling of Russia-friendly President Viktor Yanukovych.

European military observers who were held more than a week by insurgents in Slovyansk walked free Saturday. But the city's self-declared "people's mayor" — Vyacheslav Ponomarev — has boasted that he holds an unspecified number of other captives. The prisoners are believed to include Ukrainian journalists, activists and politicians. Ukrainian authorities have repeatedly claimed victories in capturing checkpoints surrounding Slovyansk, although such boasts have often proven overstated.

Government buildings have been seized by pro-Russian groups in more than a dozen or so cities and town across eastern Ukraine.

The goals of the eastern pro-Russian insurgency are ostensibly geared toward pushing for broad powers of autonomy. Russia, which the international community has accused of promoting the unrest, has vociferously condemned recent Ukrainian security operations in the east.

The self-styled Donetsk People's Republic says it plans to hold a referendum on autonomy by May 11, but with less than a week remaining, little visible effort has been made to arrange for a vote.

Al Jazeera and The Associated Press