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
Russian President Vladimir Putin has expressed his "utmost concern" over the escalation of violence in Ukraine during conversations with David Cameron and German Chancellor Angela Merkel.
The Kremlin said in a statement: "Vladimir Putin stressed the critical importance of an immediate end to bloodshed, the need to take urgent measures to stabilise the situation and suppress extremist and terrorist attacks."
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.