Calls are mounting in Ukraine to put Yanukovych on trial, after a tumultuous presidency in which he amassed powers, enriched his allies and family and cracked down on protesters. Anger boiled over last week after government snipers killed scores of protesters in the bloodiest violence in Ukraine's post-Soviet history.
The turmoil has turned this strategically located country of 46 million inside out over the past few days, raising fears that it could split apart. The parliament speaker is now nominally in charge of a country whose failing economy is on the brink of default and whose loyalties are sharply torn between Europe and longtime ruler Russia.
"The state treasury has been torn apart, the country has been brought to bankruptcy," Arseniy Yatsenyuk, a protest leader and prominent lawmaker whose name is being floated as a possibility for prime minister, said in parliament Monday.
Ukraine's acting finance minister said Monday that the country needs $35 billion (25.5 billion euros) to finance government needs this year and next and expressed hope that Europe or the United States would help.
Arsen Avakhov, the acting interior minister, said on his official Facebook page Monday that a warrant has been issued for the arrest of Yanukovych and several other officials for the "mass killing of civilians."
At least 82 people, primarily protesters, were killed in clashes in Kiev last week.
After signing an agreement Friday with the opposition to end a conflict that had turned deadly, Yanukovych fled the capital of Kiev for eastern Ukraine. Avakhov said he tried to fly out of Donetsk but was stopped, then went to Crimea on Sunday.
Yanukovych freed his official security detail and then drove off to an unknown location, turning off all forms of communication, Avakhov said.
"Yanukovych has disappeared," he said.
Security has been tightened across Ukraine's borders, the Interfax news agency quoted the State Border Guard service as saying.
Avakhov published a letter that he said was from Yanukovych, dated Monday, in which he gives up his security guard. Yanukovych's aides and spokespeople could not be reached Monday to verify the reported letter - they have been rapidly distancing themselves from him as his hold on power disintegrates.
Tensions have been mounting in Crimea in southern Ukraine, where pro-Russian protesters gathered in front of city hall in the port of Sevastopol on Monday chanting "Russia! Russia!"
Russia maintains a large naval base in Sevastopol that has strained relations between the countries for two decades.
"Extremists have seized power in Kiev and we must defend Crimea. Russia must help us with that," said Anataly Mareta, head of a Cossack militia in Sevastopol.
The head of the city administration in Sevastopol quit Monday, and protesters replaced a Ukrainian flag near the city hall building with a Russian flag.
Russian President Vladimir Putin's position on the turmoil in Ukraine will be crucial to the future of Crimea and to Ukraine. Putin has not spoken out about the events in Ukraine recently.
Putin did speak with German Chancellor Angela Merkel by telephone Sunday and the German government said the two agreed that Ukraine's "territorial integrity must be respected." They said Russia and Germany share a common interest in a politically and economically stable Ukraine.
On Monday, German government spokesman Steffan Seibert told reporters that Ukraine's new leaders should consider the interests of the south and east - the pro-Russian sections of Ukraine - in the composition of a new government. He also repeated that the offer of an association agreement with the EU is still on the table.
EU foreign policy chief Catherine Ashton was meeting Monday with protest movement leaders who are now de facto in charge of Ukraine.
Yanukovych set off a wave of protests by shelving an agreement with the European Union in November and turning toward Russia. Within weeks, the protest expanded to include outrage about corruption and human rights abuses and calls for Yanukovych's resignation.
Activist Valeri Kazachenko said Yanukovych must be arrested and brought to Kiev's main square for trial.
"He must answer for all the crimes he has committed against Ukraine and its people," he said as thousands continued to flock to the area to light candles and lay flowers where dozens were shot dead during clashes with police last week. "Yanukovych must be tried by the court of the people right here in the square."
Yet Yanukovych has proved politically resilient, rising to top posts in Ukrainian politics despite two runs-in with the law during his youth for assault and robbery. In Ukraine's 2004 Orange Revolution which pressed for democratic reforms, his fraud-ridden victory in presidential elections was overturned. He soon came back as prime minister and then was elected president in 2010, riding on a wave of popular disappointment in the squabbling Orange team.
As president, Yanukovych moved quickly to consolidate power and wealth, curb free speech and oversee the imprisonment of his top political rival, former Prime Minister Yulia Tymoshenko. But as protesters took control of the capital over the weekend, many allies turned against him, concerned for their own political survival.
With Yanukovych nowhere to be found, parliament speaker Oleksandr Turchinov assumed presidential powers. Turchinov said Monday that he hopes to form a coalition government by Tuesday.
But emotions are running high among the country's rival parties. When a leading member of Yanukovych's party, Oleksandr Efremov, told parliament Monday that he was crossing over to the opposition, an opposition lawmaker showered him with insults.
Meanwhile, Yanukovych's archrival Tymoshenko, the blond-braided heroine of the 2004 Orange Revolution, is back on the political scene after having been freed from prison.
Ukraine's acting Finance Minister Yuri Kolobov said Monday that Ukraine hopes for an emergency loan within the next two weeks and called for an international donors conference to discuss aid to Ukraine. The U.S. has said is ready to help Ukraine get aid from the International Monetary Fund.
The protest movement has been in large part a fight for the country's economic future - for better jobs and prosperity. Ukraine's trading partners are also interested in its large potential consumer market, educated workforce, significant industrial base and good natural resources, in particular rich farmland.
But Ukraine itself has struggled with corruption, bad government and short-sighted reliance on cheap gas from Russia. Political unrest has pushed up the deficit and sent exchange rates bouncing, and may have pushed the economy back into a recession.
Per capita economic output is only around $7,300, even adjusted for the lower cost of living, compared to $22,200 in Poland and around $51,700 in the United States.
Danilova reported from Kiev. Angela Charlton and Jim Heintz in Kiev contributed to this report.