Thousands of pro-Russian protesters have assaulted Odessa’s police headquarters, days after deadly clashes and a fire there killed dozens of their comrades in what Kiev charged was a Russian plot to “destroy Ukraine”.

The unrest in the southern port city threatened a new front in the Ukrainian government’s battle against pro-Moscow militants, with an expanded military operation under way in the east against gunmen holding more than a dozen towns.

Prime Minister Arseniy Yatsenyuk said Russia was executing a plan “to destroy Ukraine and its statehood”.

He was in Odessa to observe mourning for the 42 people who died there in clashes and the fire on Friday – most of them pro-Russian militants.

The unrest shaking the Black Sea city of one million people, he said, aimed “to repeat in Odessa what is happening in the east of the country”.

In an effort to head off any retribution on the streets for Friday’s bloodshed, Yatsenyuk sacked Odessa’s police chiefs and ordered an inquiry.

The under-attack police in the headquarters also released some of the 150 pro-Russian militants arrested in Friday’s clashes.

Although Moscow has admitted sending troops into Crimea ahead of annexing the strategic peninsula in March, it denies having a hand in Ukraine’s unrest in the east and in Odessa.

Instead it blames the Kiev government and its Western backers for the carnage.

Moscow has also demanded a halt to the Ukrainian military offensive in the east, saying it has received “thousands” of calls for help from the population there for it to intervene.

Tens of thousands of Russian troops have been parked on Ukraine’s border for two months, ready for an invasion Russian President Vladimir Putin says he has a right to launch – but “hopes” he won’t have to.

Ukrainian officials have pushed on regardless with the operation, determined to crush the pro-Kremlin rebels.

They have also put the armed forces on “combat alert” and brought back conscription as the risk of invasion looms.

The three-day death toll from the eastern offensive meanwhile stood at 10 at least – half of them servicemen – as soldiers confronted gunmen in towns around the rebel bastion of Slavyansk.

Sporadic fighting was also reported overnight in the eastern city of Lugansk and the port city of Mariupol.