The Ukrainian Parliament, which voted to remove President Viktor Yanukovych from office, named Speaker Oleksandr Turchynov as interim president Sunday.
The vote to dismiss Yanukovych followed days of bloody protests in which at least 88 people were killed -- protests touched off by his rejection of a trade pact with the EU.
Turchynov told Parliament it had until Tuesday to form a new unity government.
"I want to make Ukraine a modern European country," he said Sunday in a televised address. "If I can do that through the president's position, I will do my best.
"We have to return to the family of European countries," he said.
Catherine Ashton, the EU foreign policy chief, said she would travel to Kiev Monday to discuss economic stabilization and EU support "for a lasting solution to the political crisis."
U.S. officials said Sunday Washington sides "with the Ukrainian people" and called the Parliament's decisions legitimate, cautioning against any military response by Russia, the BBC reported.
Russian Foreign Minister Sergei Lavrov said the opposition in Kiev had "in effect seized power" through violence. Russia has recalled its ambassador to Ukraine.
Parliament voted to seize Yanukovych's luxury estate near Kiev, which protesters entered Saturday, the BBC reported.
Yanukovych, who has refused to resign, has abandoned his estate and was been blocked from taking a plane to Russia, RIA Novosti reported.
Border Service spokesman Serhiy Astakhov said the plane did not have the documents required to cross the border.
Yanukovych then got into an armored car and drove away.
He has left Kiev for Kharkiv, where he is meeting with his supporters, presidential spokeswoman Anna German said.
Former Prime Minister Mykola Azarov has successfully left for Russia, law enforcement agencies told Interfax-Ukraine.
Thousands of opposition supporters remain in Independence Square, and Ukrainian Democratic Alliance for Reforms party leader Vitali Klitschko is encouraging them to stay on the streets to oversee the exchange of power.
Klitschko said he and other leaders would consider bringing charges against Yanukovych for the deaths of dozens of protesters, the Kyiv Post reported.