A large crowd of pro-Russian separatists has stormed the regional administration building in Luhansk, one of the largest cities in Ukraine's troubled east.
Activists shouting "Referendum Russia" flew a Russian flag over the building after a number of men armed with metal bars smashed the windows and doors to gain access.
The action will doubtless raise tensions further in the east, where insurgents have seized control of police stations and other government buildings in at least 10 cities and towns.
Eastern Ukraine, with a large Russian-speaking population, was the heartland of support for Viktor Yanukovych, the Russia-friendly president who was ousted in February.
Around 3,000 demonstrators had gathered in front of the administration building as around 150 men, some masked and wielding baseball bats, charged into the premises facing little resistance. Later protesters formed a corridor for police who had been inside the building to leave.
Luhansk, a city of about 450,000 people, is just 15 miles from the Russian border.
The incident came as Ukraine's parliament in Kiev discussed the possibility of holding a national referendum on whether the country should remain a united state or a loose federation that allows regions more powers.
However, no consensus was reached on how a referendum would be phrased or when it could be held.
The incident came as the European Union released the names of 15 new targets of sanctions because of their roles in the Ukraine crisis.
The list also includes General Valery Gerasimov, chief of the Russian general staff, and Lieutenant General Igor Sergun, identified as the head of GRU, the Russian military intelligence agency.
Russia condemned what it called 'Iron Curtain' sanctions and said the EU was "under Washington's thumb".