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
The Ukrainian parliament has approved the formation of a national coalition government.
The parliament has proposed former economy minister Arseny Yatseniuk to lead the coalition as prime minister.
"Our country is on the brink of economic and political collapse," Yatseniuk told parliament after his nomination.
He warned that Ukraine faced threats to its territorial integrity, and promised to pursue European integration.
But he also warned of "unpopular" decisions ahead to tackle the crisis.
The new government follows the overthrow of President Viktor Yanukovich, who has not been confirmed as having been seen since Saturday.
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.