Ukraine's acting President Oleksander Turchinov has called an emergency meeting with the heads of security services as pro-Russian protesters stormed government buildings in a number of major cities in the country's east, calling for referenda on joining Russia.
In Donetsk, 80km west of the Russian border, a group of 50 people, including many in masks carrying sticks and stones, moved away from a crowd of 2,000 rallying the main city square on Sunday and surged into the provincial government building and smashed windows.
The security forces pulled a water canon up to the 11-story building, but did not use force against the protesters.
The activists issued an ultimatum demanding an emergency session of the regional council to start by midnight and threatening to otherwise replace it with a "people's council".
Later on Sunday, Interior Minister Arsen Avakov vowed to restore order in eastern Ukraine without using violence and also accused Ukraine's ousted president Viktor Yanukovich, whose political base was in Donetsk, of conspiring with Putin to fuel tensions.
"The people who have gathered are not many but they are very aggressive," Avakov said in a statement on his Facebook page.
"The situation will come back under control without bloodshed. 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."
In the eastern city of Lugansk, police were forced to fire tear gas at a few hundred protesters who tried to storm the local security service building in order to win the release of 15 pro-Russian activists arrested earlier in the week.
An AFP news agency reporter saw several men break down the door of the security service building while others pelted its windows with stones and eggs.
A police officer and a demonstrator were injured in the disturbances.
The tensions prompted the region's governor to rush to the scene for negotiations and try to talk those who managed to enter the building into giving themselves up to police.
Several more peaceful protests, assembling about 2,000 people, were also held in the eastern city of Kharkiv.
But an AFP reporter saw several hundred protesters surround a group of 15 ultra-nationalist Right Sector group members and force them to march down a steep hill on their knees in what appeared to be a humiliation ritual that drew no police response.
The unrest and threat of bloodshed has sparked concern in both Kiev and Western countries that Russian President Vladimir Putin may order his troops into the eastern regions following his promise to "protect" his compatriots there.
Washington believes that Russia has massed about 40,000 soldiers near the eastern border of Ukraine.
Moscow has denied plans to move its troops beyond Crimea but has thus far pulled only a few hundred troops back from the border region.