Hundreds of pro-Russian activists stormed several state buildings in three east Ukrainian cities on Sunday, demanding a referendum on joining Russia and further intensifying tensions between the two countries.
Waving Russian flags, the protesters made their way into regional government buildings in the industrial city of Donetsk, security service offices in Luhansk, and seized the regional administrative building in Kharkiv, Reuters reports.
Several activists broke away from a crowd of 2,000 rallying on Lenin Square in the centre of Donetsk and stormed the 11-storey regional administration building, in what was billed as the day's most violent protest.
They then hung a Russian flag from a second-floor balcony, while hundreds waiting outside cheered and chanted 'Russia, Russia', a Reuters source claims.
Donetsk's local police spokesman Ihor Dyomin said: "Around 1,000 people took part (in the storming of the building), mostly young people with their faces covered."
Participants in a similar demonstration in nearby Luhansk demanded the release of pro-Russian activists arrested by the Ukrainian security services in recent days on charges of plotting violent unrest and independence from Kiev.
Blaming ousted Ukrainian President Viktor Yanukovych and Russian President Vladimir Putin for the unrest, Ukrainian Interior Minister Arsen Avakov said the situation will be brought under control without any violence.
"The police will not shoot people due to a bunch of paid provocateurs," Avakov said. "That is the order to law enforcement officers, it's true. But the truth is that no one will peacefully tolerate the lawlessness of provocateurs."
The eastern Ukrainian cities have witnessed several demonstrations by pro-Russian activists in recent weeks, raising fears of an invasion by Moscow, which has already committed to safeguarding the Russian-speaking population in the region.
The US and its European allies claim that Russia, who annexed the Crimean peninsula from Ukraine last month, has accumulated around 40,000 soldiers near the eastern border of Ukraine.