Protesters have seized the Kiev office of President Viktor Yanukovich and the opposition has demanded a new election be held by May, as the pro-Russian leader's grip on power eroded and his whereabouts were unknown.
Anti-government demonstrators entered Yanukovich's compound in the capital on Saturday and were controlling the entrance, a Reuters news agency journalist at the scene reported.
Security guards were present inside the building but were not trying to expel the protesters.
The president's residence outside the capital appeared to have been abandoned. Local media said protesters entered the sprawling grounds but it was unclear whether they were inside the building. Interfax, a Russian news agency, said some security guards were present.
Ukrainian opposition leader Vitaly Klitschko earlier on Saturday said that Yanukovich had left Kiev and that the country must hold early elections.
"Today he left the capital," Klitschko told an emergency session of parliament, which was debating an opposition motion calling on Yanukovich to resign.
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"Millions of Ukrainians see only one choice - early presidential and parliamentary elections."
Yanukovich had been due to visit the northeastern city of Kharkiv. His residence outside the capital was empty and unguarded and journalists were entering freely, local media reported.
Parliament was sitting on Saturday in the wake of a deal to end days of carnage in the capital.
Thousands of protesters on Kiev's Independence Square are demanding Yanukovich go immediately, sceptical of a European Union-brokered accord under which the embattled leader agreed to give up powers, hold early elections by the end of the year and form a government of national unity.
The UDAR (Punch) opposition party of Klitschko submitted the parliamentary resolution calling on Yanukovich to quit to clear the way for early elections.
Lawmakers elected a close ally of jailed opposition leader Yulia Tymoshenko to the powerful post of parliament speaker on Saturday, replacing a loyalist of Yanukovich.
Oleksander Turchynov is a senior member of Tymoshenko's Fatherland party. He was elected by 288 votes in the 450-seat parliament.
The crisis began with protests in November after Yanukovich turned his back on a far-reaching economic deal with the European Union in favor of closer ties with Russia instead.
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 Russian aid to pay its debt.
Addressing the crowd on Independence Square earlier, Klitschko said he would seek support from lawmakers "to get rid of" Yanukovich.
Protesters cheered and chanted "Bandits out!"
The EU-brokered deal followed two days of violence that turned central Kiev into a war zone and left at least 77 people dead.
"I believe parliament today will be dissolved and Yanukovich will be ousted," said 58-year-old protester Vasyl Lubarets.
As the parliament sitting opened, the pro-Yanokovich speaker of the assembly, Volodymyr Rybak, said he was standing down due to ill-health.