More than 60 people arrested over Friday's violence in the Ukrainian port city of Odessa have been freed by police after protesters attacked the main police station.
Several hundred pro-Russian protesters besieged the police headquarters, breaking through windows and doors.
The clashes on Friday left more than 40 dead, mostly pro-Russian separatists killed in a building fire.
Ukraine's interim PM has accused police of failing to prevent Friday's unrest.
The mood has hardened in the towns around Sloviansk. Many people are horrified by Friday's deaths of dozens of pro-Russian activists in Odessa, something that they say "cannot be forgiven".
But even in this Russian-speaking heartland, there are occasional signs of support for a united Ukraine: the odd blue and yellow national flag poking from a window, or those colours painted onto a lamppost. But the overwhelming mood is defiance and anger at Kiev for sending troops here.
Outside the police station-turned rebel HQ in Konstantinovka men were making Molotov cocktails in beer bottles. Locals brought carrier bags full of food, for those inside. And a crowd gathered, demanding to know how they could defend their town, asking for weapons. There are a lot more barricades than when the Ukrainian military operation began. The pro-Russian groups want to slow the advance of any troops - even if they can't stop them.
Arseniy Yatsenyuk has ordered a full investigation, and said the violence was "part of a plan fomented by Russia to destroy Ukraine".
"Russia's aim was to repeat in Odessa what is happening in the east of the country," he said.
Ukrainian troops are carrying out operations in the east to wrest control of government buildings from pro-Russia separatists.
The troops have encircled Sloviansk, where pro-Russians appear to control the centre.
But Yatsenyuk insisted Kiev had not lost control entirely.
He said much would depend on whether local populations "support peace and stability or whether they support those who are sponsored by Russian".
On Friday, running street battles involving petrol bombs led to a fire breaking out on the third floor of a trade union building.
Dozens of pro-Russian activists had barricaded themselves inside. While some were rescued, 38 people either burned, suffocated or jumped to their deaths. Others died from gunshot wounds in the clashes.
As Odessa observed a day of mourning on Sunday, hundreds of pro-Russian activists marched on the police station demanding those arrested over the clashes were released.
The initially peaceful gathering turned violent as protesters - some wearing masks and carrying improvised weapons - broke windows and forced the gates.
Police then released 67 detainees, to cheers from the crowds of "Russia, Russia" and "Russians won't abandon their own".
The prosecutor's office later accused the police officers involved of "blatant disregard for their duties", and said it had had no part in ordering the releases.
In diplomatic developments on Sunday, Russia's President Vladimir Putin and German Chancellor Angela Merkel spoke by telephone.
The Kremlin said they had agreed on the importance of further dialogue, and that the president of the Organisation for Security and Cooperation in Europe (OSCE), Didier Burkhalter, would visit Moscow on Wednesday for talks.
Ukraine's pro-Moscow President Viktor Yanukovych was overthrown by pro-Western protesters in February.