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
At least 21 people have been killed in Kiev in violent clashes between police and protesters as the overnight truce agreed by the government and opposition activists broke down.
The two-day death toll now stands at around 43 in the bloodiest violence seen since Ukraine was declared an independent state 22 years ago.
A Reuters photographer counted 21 bodies in civilian clothes around Independence Square.
Activists have thrown petrol bombs and rocks at officers, while both sides have accused each other of using live ammunition.
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.