Protesters seized the Kiev office of President Viktor Yanukovich on Saturday and his whereabouts were a mystery, as the pro-Russian leader's grip on power rapidly eroded following bloodshed in the Ukrainian capital.
Parliament 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.
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-defence'' militia of protesters.
The quick disintegration of Yanukovich's government marked a setback for Russian President Vladimir Putin, who had counted on the Ukrainian leader to bring Ukraine into a Eurasian Union to help rebuild as much of the old Soviet Union as possible.
A senior security source said Yanukovich was still in Ukraine but was unable to say whether he was in Kiev.
An ally was quoted as saying he was in a city in the country's generally pro-Russian east.
The government, still led by a Yanukovich ally, said it would ensure a smooth handover of power to a new administration.
''RESPONSIBLE TRANSFER OF POWER''
''The cabinet of ministers and ministry of finance are working normally,'' the government said in a statement.
''The current government will provide a fully responsible transfer of power under the constitution and legislation.'
'Yanukovich, who enraged much of the population by turning away from the European Union to cultivate closer relations with Russia three months ago, made sweeping concessions in a deal brokered by European diplomats on Friday after days of pitched fighting in Kiev, with police snipers gunning down protesters.
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 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.
''Today he left the capital,'' opposition leader Vitaly Klitschko, a retired world heavyweight boxing champion, told an emergency session of parliament about the president.''Millions of Ukrainians see only one choice - early presidential and parliamentary elections.''
Klitschko then tweeted that an election should be held no later than May 25.The senior security source said of Yanukovich: ''Everything's ok with him ... He is in Ukraine.''
Asked whether the leader was in Kiev, the source replied: ''I cannot say.''
The UNIAN news agency cited Anna Herman, a lawmaker close to Yanukovich, as saying the president was in the northeastern city of Kharkiv, in a mainly Russian-speaking province.
Two protesters in helmets stood at the entrance to the president's Kiev office. Asked where state security guards were, one, who gave his name as Mykola Voloshin, said: ''I'm the guard now.''
Dmytro Pylipets, 32, a doctor from Kharkiv who was wearing military fatigues and helmet, said: ''I think Yanukovich is frightened and panicking. I feel we are almost there. The Maidan revolution is almost done.''
POSSIBLE TYMOSHENKO COMEBACK
The release of Tymoshenko would transform Ukraine by giving the opposition a single leader and potential future president, although Klitschko and others also have claims.
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.In a sign of the quick transformation, the interior ministry responsible for the police swung behind the protests.
It said it served ''exclusively the Ukrainian people and fully shares their strong desire for speedy change''.
New Interior Minister Arsen Avakov told Ukraine's Channel 5 TV:
''The organs of the Interior Ministry have crossed to the side of the protesters, the side of the people.''
However, underscoring Ukraine's regional and ethnic divisions, leaders of Russian-speaking eastern provinces voted to challenge anti-Yanukovich steps by the central parliament.
Eastern regional bosses meeting in Kharkiv adopted a resolution saying parliament's moves ''in such circumstances cause doubts about their ... legitimacy and legality.
''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.''
Kharkiv Governor Mikhaylo Dobkin told the meeting: ''We're not preparing to break up the country. We want to preserve it.''
Yanukovich's broad concessions on Friday ended 48 hours of violence that had turned the centre of Kiev into an inferno of blazing barricades.
Without enough loyal police to restore order, the authorities had resorted to placing snipers on rooftops who shot demonstrators in the head and neck.
The foreign ministers of France, Germany and Poland negotiated the concessions from Yanukovich, in what the Kremlin's envoy acknowledged as superior diplomacy.
''We joined the talks later, which wasn't very right,'' Russian envoy Vladimir Lukin was quoted as saying by Interfax.
Yanukovich, 63, a burly former Soviet regional transport official with two convictions for assault, did not smile during a signing ceremony at the presidential headquarters on Friday.
''YOU'LL ALL BE DEAD''
It took hard lobbying to persuade the opposition to accept the deal, and crowds in the streets made clear they were not satisfied with an arrangement that would leave Yanukovich in power.
Video filmed outside a meeting room during Friday's talks showed Polish Foreign Minister Vladislaw Sikorski pleading with opposition delegates: ''If you don't support this, you'll have martial law, you'll have the army, you'll all be dead.''
Anti-government protesters remained encamped in Independence Square, known as the Maidan or ''Euro-Maidan'', through the night.
They held aloft coffins of slain comrades and denounced opposition leaders for shaking Yanukovich's hand.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.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.