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 US has warned Russia over their military exercises along the border with Ukraine, saying they should not take any steps that could be misinterpreted or "lead to miscalculation".
"I'm closely watching Russia's military exercises along the Ukrainian border," U.S. defence secretary Chuck Hagel said at a NATO news conference.
"I expect Russia to be transparent about these activities, and I urge them not to take any steps that could be misinterpreted, or lead to miscalculation during a very delicate time - a time of great tension."
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.