Protesters seized the Kiev office of President Viktor Yanukovich on Saturday and the opposition demanded a new election be held by May, as the pro-Russian leader's grip on power rapidly eroded following bloodshed in the capital.
Protesters entered the president's compound in the capital and were controlling the entrance, a Reuters reporter said at the scene. Security guards were present inside the building but were not trying to expel the protesters.
Yanukovich's residence outside the capital was also abandoned, and journalists were freely able to enter, local media reported.
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A security source said the president was still in Ukraine but was unable to confirm whether he was in Kiev.
Yanukovich, who enraged much of the population by turning away from the European Union to build closer ties with Russia three months ago, made sweeping concessions in a deal brokered by European diplomats on Friday after days of violence that killed 77 people.
But the deal, which called for early elections by the end of the year, was not enough to satisfy demonstrators who want him out immediately after bloodshed that saw his police snipers shooting from rooftops.
The speaker of parliament, a Yanukovich loyalist, resigned. Parliament has quickly acted to implement the deal, voting to restore a constitution curbing the president's powers and change the legal code possibly allowing his arch-foe, jailed opposition leader Yulia Tymoshenko, to go free.
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.
"Today he (Yanukovich) left the capital," opposition leader Vitaly Klitschko, a retired world heavyweight boxing champion, told an emergency session of parliament debating an opposition motion calling on the president to resign.
"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 the capital, 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.
At the president's office in the capital, 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."