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 have vowed to remain in Independence Square in Kiev "until victory" after a night of bloody violence led to the deaths of 25 people.
Protesters were reportedly using pickaxes to tear up cobblestones before hurling them at police in Independence Square, which is also known locally as The Maidan.
"They can come in their thousands but we will not give in. We simply don't have anywhere to go. We will stay until victory and will hold the Maidan until the end," a 44-year-old, who only have his first name of Volodymyr, said.
Witnesses say the square now resembles a battleground with flames and black smoke coming from a trade union building used as an anti-government headquarters.
Another 36-year-old activist called Vitaly said: "We will stay until victory. We want our children to grow up in a normal country where there are civilised laws not the laws of a prison colony."
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.