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
Twenty-five people have been killed in the Ukrainian capital Kiev, in the deadliest clashes since anti-government demonstrations began three months ago.
What caused the original unrest?
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.