DONETSK, Ukraine (AFP) - About 50 pro-Russian protesters chanting "Donetsk is a Russian city!" broke through police lines on Sunday and stormed inside the main administration building of the eastern Ukrainian city.
The activists moved away from a crowd of about 2,000 rallying on the main city square and threw firecrackers at police surrounding the government seat before raising the Russian flag above the 11-storey building.
An AFP reporter at the scene said security officers pulled a water cannon up to the building but did not use force against the activists. The protesters ripped away several of the police officers' shields and some were seen pelting them with Russian flag poles that had been waved by many at the protests.
The action began earlier in the day with calls for Donetsk - a heavily Russified industrial city of one million people that has witnessed weeks of similar Sunday rallies - to stage an independence referendum like the one that led to Crimea's annexation by Russia last month. Some in the crowd chanted "Give us a referendum" and "Nato go home".
Police in the eastern city of Lugansk 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.
The region's security service had accused the group of planning to seize the main administration building on Thursday with "the use of arms and explosives".
An AFP reporter saw several men break down the door of the security service building while others pelted its windows with stones and eggs.
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 to give themselves up to the 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 southern and eastern regions of the culturally splintered nation of 46 million have been hit by waves of at times deadly protests that followed the February 22 fall in Kiev of pro-Russian president Viktor Yanukovych and the rise of a team that is seeking closer ties to the West.
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.