Several hundred pro-Russia protesters have stormed the Ukrainian naval headquarters in the city of Sevastopol in the Crimean peninsula.
They had been holding a rally outside the building early on Wednesday morning.
They broke through the fence, forcing their way in, and replaced Ukrainian flags with Russian ones.
A spokesperson for the Ukrainian ministry in Crimea said some of the men and women were masked but not armed.
On Tuesday, Sevastopol signed a treaty with Moscow to become part of the Russian Federation. The city is home to Russia’s large Black Sea fleet.