Ukraine's parliament voted to remove President Viktor Yanukovych 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.
Yanukovych 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.
His precise whereabouts on Sunday were still unknown. But residents in the eastern town of Donetsk, his stronghold, said that security had been reinforced along the main approach road to his private home in the town, suggesting he might be there.
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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 Yanukovych should pull Ukraine 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 Yanukovych to deliver Ukraine as a central member.
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Members of the Ukrainian parliament, who abandoned Yanukovych 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 eastern city of Kharkiv, Yanukovych 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.
The 63-year-old Yanukovych, who looked pale and tired, said he would not sign any draft laws passed by the parliament into force. "I will not sign anything with bandits who terrorise the country," he said.
Interfax news agency said border guards refused to let Yanukovych exit the country when he tried to take an unscheduled charter flight from Donetsk and he was eventually driven away by his own security guards.
"We have a legitimate, living president. We just don't know where he is," Oleh Tsaryov, a member of Yanukovych's Party of regions, said grimly on Russian television.
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At Yanukovych's abandoned secret estate a short distance from Kiev, people flocked to take photographs of his private zoo with ostriches and deer, replica ancient Greek ruins, and lavish waterways and follies.
Despite Yanukovych's defiance, the dismantling of his authority seemed all but complete. His cabinet promised a transition to a new government, the police declared themselves behind the protesters and his arch-rival Tymoshenko went free.
Tymoshenko, with her trademark braided hair, waved to supporters from a car as she was driven out of the hospital in Kharkiv, where she has been treated for a bad back while serving a seven-year sentence since 2011.
Setting herself immediately on a collision course with Moscow, Tymoshenko said she was sure her country would join the European Union in the near future.
Economic and Monetary Affairs Commissioner Olli Rehn said on Sunday the EU was ready to offer "substantial financial assistance" once Ukraine had a new government.
"From a European point of view it is important that we provide a clear European perspective for the Ukrainian people who have shown their commitment to European values," Rehn said after a meeting of the world's financial leaders in Sydney.
Russian Foreign Minister Sergei Lavrov said "illegal extremist groups were refusing to disarm and in fact are taking Kiev under their control with the connivance of opposition leaders".
The White House said Washington was keen to see the country build a new government and hold early elections and welcomed Tymoshenko's release.
Thousands of opposition supporters stayed on Kiev's Independence Square, scene of nearly three months of protests, overnight singing patriotic songs and musing on the rollercoaster events.
Earlier, coffins - some of them open showing the victim face up - were displayed in front of the crowd as priests said prayers. People crossed themselves in front of makeshift shrines with candles and pictures of the dead.
Two captured water cannon trucks were parked in the square, like trophies of war.
Carried on to a stage in a wheelchair, an emotional and tired-looking Tymoshenko told the protesters on the square, known as the Maidan: "You have no right to leave the Maidan ... Don't stop yet."
Showing glimpses of the fiery oratory that drove her to power, Tymoshenko shouted: "This is a Ukraine of different people. The ones who died on Maidan are our liberators, our heroes for centuries."
The response was mixed. Tymoshenko is a divisive figure in Ukraine, where many have become disillusioned with a political class they see as corrupt and elitist.
Small pockets of the crowd clapped and sang Tymoshenko's name, but the chants did not catch on. Whistles could be heard. Others listened silently.
Elsewhere in the country large-scale fighting broke out when "EuroMaidan" activists were attacked by a pro-Yanukovych group, but the new provisional interior minister, Arsen Avakov, was later quoted as saying the situation was under control. TV reported clashes too in the Black Sea port of Kerch.
Earlier, the Ukrainian cabinet said it was committed to a responsible transfer of power. Military and police leaders said they would not get involved in any internal conflict.
Yanukovych enraged much of the population by turning away from the European Union to cultivate closer relations with Russia three months ago. On Friday, he made sweeping concessions in a deal brokered by European diplomats after days of street battles during which police snipers gunned down protesters.
But the deal, which called for early elections by the end of the year, was not enough to satisfy pro-Europe demonstrators on Independence Square. They wanted Yanukovych out immediately in the wake of the bloodletting.
The release of Tymoshenko transforms Ukraine by giving the opposition a single leader who may become president, although Klitschko and others also have claims.
Tymoshenko, 53, was jailed by a court under Yanukovych over a natural gas deal with Russia she arranged while serving as premier before he took office.
The EU had long considered her a political prisoner, and her freedom was one of the main demands it had for closer ties with Ukraine during years of negotiations that ended when Yanukovych turned towards Moscow in November.
She had served as a leader of the "Orange Revolution" of mass demonstrations that overturned a fraudulent election victory for Yanukovych in 2004, but after a divisive term as prime minister she lost to him in an election in 2010.