Ukraine's acting president has warned Russia that troop movements outside its naval base in Crimea would be seen as an "act of aggression". Armed men seized regional government headquarters and parliament and the Russian flag is flying from buildings
Ukrainian anti-government protesters are holding 67 policemen hostage in Kiev, the country's interior ministry said.
A statement posted on the ministry's website said: "To free the hostages police have the right to use their weapons."
Riot police had been armed with combat weapons in a bid to regain control of violent protests in which at least 50 people have died since Tuesday.
A further three policemen were unaccounted for, it said.
Ukraine's interim President has expressed concerns about signs of separatism as opposition in Russian-speaking regions grows.
Hundreds of Ukrainians and journalists flocked to the presidential palace, as reports emerged that President Yanukovich had left the city.
Vitali Klitschko was met with boos from anti-government protesters, as he attempted to convince them a peace deal is best course of action.