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- Outrage over the deaths of pro-Russian activists in riots in Odessa triggered new violence Sunday in the Black Sea port, where a mob of protesters stormed police headquarters and freed dozens of their jailed allies.

The activists had been jailed for their involvement in clashes Friday that killed more than 40 people - some died from gunshot wounds, but most from a fire that broke out in a trade union building. It was the worst violence in the Ukrainian crisis since more than 100 people died in Kiev in February, most of them shot by snipers.

Ukrainian Prime Minister Arseniy Yatsenyuk visited Odessa on Sunday to try to defuse the mounting tensions and hinted strongly that he saw Moscow's hand in the unrest spreading through southeastern Ukraine.

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 Moscow ultimately aims to take control of a huge swath of southeastern Ukraine from Trans-Dniester to Russian-dominated industrial areas in 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 to protect his country's interests.

Alexei Pushkov, a prominent member of Russia's parliament who often expresses Kremlin views on foreign policy, suggested Ukraine was destined to be split apart.

'Through the justification of arson, military operations and the killing of Russians in Ukraine, the Kiev government is destroying the basis for the existence of a united country,' Pushkov said on Twitter.

Yatsenyuk said Odessa police were being investigated for their failure to keep the peace during the riots and said he had ordered prosecutors to find 'all instigators, all organizers and all those that under Russian leadership began a deadly attack on Ukraine and Odessa.'

Hours later, however, the police bowed to a mob of several hundred pro-Russian demonstrators who attacked their headquarters, smashing doors, windows and security surveillance cameras. Shortly after some of them managed to break into an inner courtyard, police released the detainees, who were swept up by the cheering, rain-dampened crowd that had been chanting 'Freedom!'

The Interior Ministry said 67 activists had been released on prosecutors' orders. Prosecutors, however, later said they had nothing to do with the release and accused the police of failing to carry out their duties. It was not immediately clear whether any activists were still being held.

Putin spoke by telephone Sunday night with German Chancellor Angela Merkel, the latest in a series of discussions they have had about Ukraine. The Kremlin said they agreed on the importance of the role to be played by the Organization for Security and Cooperation in Europe, and said Swiss President Didier Burkhalter, whose country currently chairs the OSCE, would visit Moscow on Wednesday.

The interim government in Kiev, which took power in February, has renewed its push in recent days to quell the pro-Russian insurgency in the east, where government buildings have been seized in more than a dozen cities and towns.

Interior Minister Arsen Avakov said on his Facebook page that an 'anti-terrorist operation' was being executed in the eastern city of Kramatorsk, the latest flashpoint for unrest.

A standoff Saturday in Kramatorsk culminated with pro-Russian insurgents setting buses ablaze to ward off attacks. Russian state TV reported 10 deaths, including two among government forces, during clashes there so far. The figures could not be independently confirmed.

By midday Sunday, however, there was little sign of movement, from either government or the insurgents. The burned-out trolleybuses and a minibus lay in the road untouched.

Both sides in Ukraine's conflict have traded bitter recriminations over Friday's deaths from the Odessa rioting.

The violence began with street fighting between supporters of Ukrainian unity and activists who support Russia, with at least three people were reported killed by gunfire. The government opponents took refuge in a large trade union building, which then caught fire as opposing sides hurled Molotov cocktails at each another.

Odessa Police Chief Petr Lutsyuk issued a statement Saturday calling for calm in the city of about 1 million, but hours later he was fired by the interior minister.

The U.S. ambassador to Ukraine, Geoffrey Pyatt, said he was disturbed by signs that some of the police in Odessa may have been complicit in allowing the violence to get out of control.

'That's something which Prime Minister Yatsenyuk spoke to today,' Pyatt said on CNN's 'State of the Union.' ''And I see that he's already brought some major changes in the security leadership there in Odessa, which I think reflects the deep concern about the role that the security establishment played in Friday's violence.'

Late Sunday, about 300 supporters of the Kiev government gathered outside the regional police headquarters to demand that the armed pro-Russian demonstrators be punished. Carrying sticks and bats, they chanted, 'Glory to Ukraine!'

'The Russians bought off our police,' said Andrei Shpak, who wore a balaclava to hide his face and carried a Ukrainian flag. 'We're angry that the separatists were set free and we demand that anyone who calls for the breakup of Ukraine be punished.'

Their demands were addressed to the police chief who replaced Lutsyuk.

The victims of Friday's fire have become a rallying point by pro-Russian groups in eastern Ukraine. In a position eagerly promoted by the Kremlin, critics of the authorities in Kiev have blamed the deaths on radical ultranationalists, abetted by the government.

The activists who had been calling for unity in Ukraine say their rally Friday had come under attack by gunmen.

Kiev's efforts to quash the insurgency have focused mostly on Slovyansk, where government security forces are seeking to form a cordon around the eastern city.

It is difficult to establish how much popular support the gunmen who effectively control Slovyansk truly enjoy. The insurgency has proven hostile to supporters of the Kiev government that came to power after the toppling of Russia-friendly President Viktor Yanukovych.

European military observers operating under the auspices of the OSCE who were held captive for more than a week by insurgents in Slovyansk were freed Saturday. But the city's self-declared 'people's mayor' - Vyacheslav Ponomarev - has boasted that he holds an unspecified number of other captives. They are believed to include Ukrainian journalists, activists and politicians.

Ukrainian authorities have said repeatedly that they have taken back checkpoints surrounding Slovyansk, although such boasts have often proved to be overstated.

Andriy Parubiy, secretary of Ukraine's National Security and Defense Council, said Saturday that security sweeps would be extended beyond Slovyansk and Kramatorsk, according to the 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 pro-Russian insurgency are ostensibly geared toward pushing for broad powers of regional autonomy from Kiev. Russia has vociferously condemned Ukrainian security operations in the east, while the international community has accused Moscow of promoting the unrest.

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 organize the balloting.

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Leonard reported from Donetsk, Ukraine. Yuras Karmanau in Odessa and Lynn Berry in Moscow contributed to this report.