More than 3,000 pro-Russian protesters stormed the regional administration building in the eastern Ukraine city of Lugansk on Tuesday, AFP journalists at the scene reported.
A vanguard of around 20 youths armed with metal bars broke a window to get inside the building, which was not protected by police.
They pulled down the building's Ukrainian flag, replacing it with a Russian one, and opened the building's doors to the waiting crowd.
Inside the building's courtyard, around 100 riot police massed together in a defensive position behind their shields in a stand-off with the protesters, who did not immediately attack them.
Pro-Moscow militants in Lugansk, a city of 465,000 inhabitants near the Russian border, seized the local SBU security services building three weeks ago.