A new crisis began brewing for Ukraine on Sunday as pro-Russian separatists stormed government buildings in three eastern Ukrainian cities, demanding referendums on joining Russia.

Acting President Oleksandr Turchynov canceled a trip to Lithuania and called an emergency session with heads of the country’s security services.

A group of masked pro-Russian men in Donetsk, carrying sticks and stones, split off from a separatist rally being held at the city’s main Lenin Square and forced their way inside the regional government building, smashing windows and breaking through doors, local media reported.

Police reportedly brought a water cannon to the building, but did not use it against the protesters, who issued an ultimatum to local authorities: call an emergency session of the regional council before midnight, or they will be replaced by a “people’s council.”

In nearby Luhansk, more than 200 protesters demanding the release of 15 pro-Russian activists arrested earlier in the week clashed with police who reportedly used tear gas to try to stop them from seizing the local security service building. But the police efforts were unsuccessful, as several in the mob still managed to break down the building’s door while others pelted its windows with stones and eggs, according to an AFP reporter on the ground.

At least one police officer and a demonstrator were reportedly injured in the skirmish.

Luhansk’s regional governor reportedly entered the building later to try to negotiate with the pro-Russian crowd, encouraging them to turn themselves over to police, but to no avail.

Meanwhile, a pro-Russian crowd of some 2,000 people gathered in Kharkiv to demand that a referendum be held there. Late in the evening several of the protesters stormed the regional government building in the center of the city and raised the Russian flag atop it.

Russian flag reportedly now flying from #Kharkov regional admin bldg seized by Pro-Russian separatists pic.twitter.com/hkoW9A0v7I @EuromaidanPR

— Christopher Miller (@ChristopherJM) April 6, 2014

On Facebook, Ukrainian Interior Minister Arsen Avakov charged Russian President Vladimir Putin and deposed Ukrainian President Viktor Yanukovych of “instigating and paying for the new wave of separatist riots.”

Of those involved in the seizure of the buildings in Donetsk, he said: “There aren't many people, but the aggression is over the top. In Donetsk, to the crowd of those who stormed the building a lot of women and children were brought in. They're provoking a bloody conflict with victims.”

Orders have been given to the police in the beleaguered cities to quell the unrest “with no bloodshed,” he said, adding that those caused it will bear legal responsibility for their actions.

“A cruel approach will be used to everyone who directly conducts attacks on state buildings, on law enforcers and other civilians,” he warned.

Turchynov's press service reportedly said the acting president had called an emergency meeting with security officials in his office and took “personal control” over the escalating situation.

The turmoil comes mere weeks after Moscow seized and annexed the Black Sea peninsula of Crimea, returning it to Russia after more than six decades as part of Ukraine.

At the same time, there is concern that Putin may order his troops into the eastern regions of Ukraine under the same pretext he used to invade Crimea: ethnic Russians must be “protected" from “neo-Nazis and fascists” in Kiev who seized power in late February through a Western-backed coup d’état.

Over the past few weeks, tens of thousands of Russian troops have amassed on Ukraine’s borders and carried out war games.

The U.S. has said that Russia has placed about 40,000 soldiers near the eastern border of Ukraine, while the Kiev government estimates some 100,000 are mere miles from the country’s boundaries on all sides.

Moscow has denied that it plans to move its troops beyond Crimea and into mainland Ukraine, but its troops have not retreated from their positions at Russia’s western border.

Former Georgian President Mikheil Saakashvili wrote on Facebook amid the chaos on Sunday that what is happening in eastern Ukraine resembles the Russian moves in Crimea. He said he believes that the pro-Russian separatists will try to hold the building for three days, allowing Putin time to prepare his troops to enter Ukraine under the pretense that Kiev is not in control of the country and cannot defend Moscow’s compatriots there.

With their head government office overtaken by the masked pro-Russian separatists, some citizens of Donetsk reportedly penned an open letter to Turchynov, asking for the Ukrainian government’s help to ensure their safety.

“We ask you, using all the available means and human resources and power to take immediate measures to ensure the safety of residents of Donetsk region, to create conditions for the rights and freedoms of the people of the region, and personally oversee the performance of law-enforcement agencies of their functions and enforcement of protection of interests of Ukraine,” reads the letter.

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