The Ukrainian Parliament voted to dismiss Ukrainian President Viktor Yanukovich and set May 25 as the date for early elections after protesters seized his Kiev office on Saturday, according to Reuters.
The parliament declared Yanukovich constitutionally unable to carry out his duties on Saturday.
Deputies in the assembly stood, applauded and sang the national anthem.
The pro-Russian leader's grip on power rapidly eroded following bloodshed in the Ukrainian capital.
Parliament also voted to free his arch-rival, jailed former Prime Minister Yulia Tymoshenko. Her daughter said Tymoshenko was already free under Ukrainian law but still in the hospital where she has been held for treatment.
This comes as Yanukovich accused the opposition of staging a coup and refused to give into demands to resign, according to AFP news agency.
Leaders of mainly Russian-speaking regions of eastern Ukraine that are loyal to President Viktor Yanukovich challenged the legitimacy of the national parliament on Saturday and said they were taking control of their territories.
The move appeared to increase the possibility of a split in the sprawling former Soviet republic of 46 million, despite denials by the leaders that this was their intention.
The Kiev parliament has passed a series of measures that would reduce the president's powers and pave the way to the formation of a national unity government and early presidential elections.
Mikhaylo Dobkin, Governor of Kharkiv region in northeast Ukraine, told regional leaders meeting in the city: "We're not preparing to break up the country. We want to preserve it."
But a resolution adopted at the meeting said: "The decisions taken by the Ukrainian parliament in such circumstances cause doubts about their ... legitimacy and legality."
It added: "The central state organs are paralysed. Until the constitutional order and lawfulness are restored ... we have decided to take responsibility for safeguarding the constitutional order, legality, citizens' rights and their security on our territories."
One speaker urged the creation of civilian patrols to restore order. Another said those gathered should fear reprisals if anti-Yanukovich protesters in Kiev seize power in the whole of the country.
With people at the meeting chanting "Russia! Russia!," the atmosphere contrasted with the mood in the capital Kiev where protesters want the Moscow-backed Yanukovich to resign.
Yanukovich said he had no intention of quitting or leaving Ukraine and declared all moves taken by parliament on Saturday to be illegal and amounting to a "coup d'etat", Russian news agency Interfax reported, citing a television interview.
The newly-installed interior minister declared that the police were now behind the protesters they had fought for days, giving central Kiev the look of a war zone with 77 people killed, while central authority crumbled in western Ukraine.
At the president's headquarters, Ostap Kryvdyk, who described himself as a protest commander, said some protesters had entered the offices but there was no looting. "We will guard the building until the next president comes," he told Reuters. "Yanukovich will never be back."
The grounds of Yanukovich's residence outside Kiev were also being guarded by "self-defense" militia of protesters.
The whereabouts of Ukrainian President Yanukovich were initially unknown as some media reports said that Yanukovich was in Kharkov, a city in Eastern Ukraine, which is a stronghold of his Party of Regions.
Yanukovich made sweeping concessions in a deal brokered by European diplomats on Friday after days of pitched fighting in Kiev.
But the deal, which called for early elections by the end of the year, was not enough to satisfy demonstrators, who want Yanukovich out immediately in the wake of the bloodletting.
Parliament has quickly acted to implement the deal, voting to restore a constitution that curbs the president's powers and to change the legal code to allow Tymoshenko to go free. On Saturday, lawmakers voted to speed her release by eliminating a requirement that the president approve it.
The release of Tymoshenko would transform Ukraine by giving the opposition a single leader and potential future president.
She was jailed by a court under Yanukovich over a natural gas deal with Russia she arranged while serving as premier. The European Union 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 Yanukovich abruptly turned towards Moscow in November.
"According to Ukrainian law my mum is already a free person," daughter Yevgenia Tymoshenko told reporters, saying she was on her way to meet her mother in Kharkiv where she has been held in hospital under treatment for back pain.
A spokeswoman for the former prime minister, 53, said that although the moves in parliament already made her a free woman, Tymoshenko had not yet been released or left the hospital.
The speaker of parliament, a Yanukovich loyalist, resigned and parliament elected Oleksander Turchynov, a close ally of Tymoshenko, as his replacement.
Events were moving at a rapid pace that could see a decisive shift in the future of a country of 46 million people away from Moscow's orbit and closer to the West, although Ukraine is near bankruptcy and depends on promised Russian aid to pay its bills.
With borders drawn up by Bolshevik commissars, Ukraine has faced an identity crisis since independence. It fuses territory integral to Russia since the Middle Ages with former parts of Poland and Austria annexed by the Soviets in the 20th century.
In the country's east, most people speak Russian. In the west, most speak Ukrainian and many despise Moscow. Successive governments have sought closer relations with the European Union but have been unable to wean their heavy Soviet-era industry off dependence on cheap Russian gas.
The past week saw central state authority vanish altogether in the west, where anti-Russian demonstrators seized government buildings and police fled. Deaths in Kiev cost Yanukovich the support of wealthy industrialists who previously backed him.
Russian President Vladimir Putin had offered Kiev $15 billion in aid after Yanukovich spurned an EU trade pact in November for closer ties with Moscow. The fate of those funds is now unclear.
Washington, which shares Europe's aim of luring Ukraine towards the West, took a back seat in the final phase of negotiations; its absence noteworthy after a senior U.S. official was recorded using an expletive to disparage EU diplomacy on an unsecured telephone line last month.
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