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
Anti-government protesters are remaining defiant amid ongoing violence in the Ukrainian capital Kiev.
"What truce? There is no truce! It is simply war ahead of us! They are provoking us. They throw grenades at us. Burn our homes. We have been here for three months and during that time nothing burned," Petro Maksimchuk, 23, said.
"These are not people. They are killers. Sanctions will not help. They all should be sent into isolation in Siberia."
At least 21 civilians are believed to have died, with the government and the opposition each accusing each other of firing live ammunition.
Serhiy, a 55-year-old from the western city of Lviv, added: "It is bad that Ukraine is already broken into two parts. In the west the police and army are with us but in the east, they are against us. It is the 'Yanukovichers' who are dividing us."
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.