Clashes in Donetsk and similar rallies in the heavily Russified cities such of Lugansk and Kharkiv provided another reminder to the untested pro-Western leaders in Kiev of the monumental task facing them after their February 22 overthrow of president Viktor Yanukovych.
The unrest comes with Ukraine's borders surrounded by Russian troops who had earlier seized the Black Sea peninsula of Crimea and the economy in tatters after decades of mismanagement and government theft.
The day's most violent protest saw nearly 100 activists move away from a crowd of 2,000 rallying on the main city square of Donetsk to storm and occupy the government seat where they raised the Russian flag.
They threw firecrackers at about 200 riot police and ripped away several of their shields before raising the Russian flag above the 11-story building.
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Police in the eastern city of Lugansk were forced to fire tear gas at a few hundred protesters who broke into 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. Local news reports said two police officers were hurt in the melee.
Several dozen people also entered the government seat in the eastern city of Kharkiv after breaking through police lines.
They thrust several Russian flags out of windows as a crowd of about 2,000 outside cheered and chanted "The police are with the people".