Kiev's parliament voted to remove President Viktor Yanukovich after three months of street protests, while his arch-rival Yulia Tymoshenko hailed opposition demonstrators as "heroes" in an emotional speech in Kiev after she was released from jail.
Yanukovich abandoned the capital to the opposition on Saturday and denounced what he described as a coup after several days of bloodshed this week that claimed 82 lives.
Supporters cheered former prime minister Tymoshenko as she left the hospital where she had been held. When she spoke later in Kiev, her reception was mixed.
Her release marks a radical transformation in the former Soviet republic of 46 million people. Removal of the pro-Russian Yanukovich should pull away from Moscow's orbit and closer to Europe.
It is also a reversal for Russian President Vladimir Putin's dream of recreating as much as possible of the Soviet Union in a new Eurasian Union. Moscow had counted on Yanukovich to deliver Ukraine as a central member.
Members of the Ukrainian parliament, who abandoned Yanukovich after this week's bloodshed, applauded and sang the national anthem after declaring him constitutionally unable to carry out his duties. An early election was set for May 25.
"This is a political knockout," opposition leader and retired world boxing champion Vitaly Klitschko told reporters.
In a television interview the station said was conducted in the northeastern city of Kharkiv, Yanukovich said he would not resign or leave the country, and called decisions by parliament "illegal".
"The events witnessed by our country and the whole world are an example of a coup d'etat," he said, comparing it to the rise of the Nazis to power in Germany in the 1930s.
Interfax news agency said border guards refused to let Yanukovich exit the country when he tried to fly out from the eastern city of Donetsk.