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 ordered military exercises for 150,000 troops.
It comes after Russian flags were raised from government buildings on Ukraine's Crimea peninsula, which armed men have seized.
The exercises follow the Russian foreign ministry's pledge to "strongly and uncompromisingly" react to any violation of the rights of Russian compatriots in Ukraine.
They are among the largest such exercises Russia has undertaken in recent years.
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.