ODESSA, Ukraine -- Hundreds of pro-Russian demonstrators stormed police headquarters in Odessa on Sunday and won the release of 67 people detained after deadly clashes in the Ukrainian port city.
More than 40 people died in the riots two days earlier, some from gunshot wounds, but most in a horrific fire that tore through a trade union building.
Ukrainian Prime Minister Arseniy Yatsenyuk, who hinted strongly that he saw Moscow's hand in the unrest spreading through southeastern Ukraine, visited Odessa on Sunday to try to defuse the mounting tensions.
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 doesn't want to send in troops but will if necessary.
Yatsenyuk said police were being investigated for their failure to maintain order and he had charged prosecutors with "finding all instigators, all organizers 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 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 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.
By midday Sunday, 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.
At least 12 government armored personnel carriers were spotted driving through the town Saturday, although they appeared to have returned to their base at a military airfield on the edge of the city by day's end.
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 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.
Efforts to counteract the insurgency have focused mostly on Slovyansk, where the Ukrainian authorities are seeking to form a security cordon around the eastern 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 forces in more than a dozen or so cities and town across eastern Ukraine.
Andriy Parubiy, secretary of Ukraine's National Security and Defense Council, said late Saturday that security sweeps would be extended beyond Slovyansk and Kramatorsk, according to Interfax-Ukraine news agency.
Traffic around the Donetsk region, where the insurgency is strongest, has been impeded by a proliferation of barricades guarded by men armed with sticks, automatic rifles and handguns.
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 to make that vote happen.
Leonard reported from Donetsk, Ukraine.