Russia has questioned the authority of Ukraine's acting government, with prime minister Dmitry Medvedev saying the country's acting authorities have come to power as a result of an 'armed mutiny'.
Ukraine's acting government has issued an arrest warrant for president Viktor Yanukovych, accusing him of mass crimes against the protesters who stood up for months against his rule.
Yanukovych himself has reportedly fled to the Black Sea peninsula of Crimea, a pro-Russian area in Ukraine.
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.
Ukraine's parliament speaker is now nominally in charge of a country whose ailing economy is on the brink of default and whose loyalties are sharply torn between Europe and long-time ruler Russia.
Russia and the European Union appear to be taking opposing sides in Ukraine's new political landscape.
Medvedev has questioned the legitimacy of the new Ukrainian authorities. He said the acting authorities have come to power as a result of an "armed mutiny", so their legitimacy is causing "big doubts".
In Brussels, European Commission spokesman Olivier Bailly referred to parliament speaker Oleksandr Turchinov as the "interim president" and said Turchinov will meet with visiting EU foreign policy chief Catherine Ashton in Kiev.
Turchinov said he hopes to form a new coalition government by Tuesday.
Ukraine's acting interior minister, Arsen Avakhov, said on his official Facebook page 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.
Yanukovych set off a wave of protests by shelving an agreement with the European Union in November and turning instead for a bailout loan from Russia. Within weeks, the protests expanded to include outrage over corruption and human rights abuses, leading to calls for Yanukovych's resignation.
After signing an agreement with the opposition to form a unity government, Yanukovych fled Kiev for his pro-Russian power base in eastern Ukraine. Avakhov said he tried to fly out of Donetsk but was stopped then went to Crimea on Sunday.
Yanukovych then freed his official security detail and 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 State Border Guard service said.
Avakhov published a letter that he said was from Yanukovych in which he gave up his security guard. Yanukovych's aides and spokespeople could not be reached to verify the reported letter.
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. "Yanukovych must be tried by the court of the people right here in the square."
Tensions have been mounting in Crimea in southern Ukraine. Russia maintains a large naval base in Sevastopol that has strained relations between the countries for two decades.
Pro-Russian protesters gathered in front of city hall in the port of Sevastopol on Monday.
"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.