Oleksandr Turchynov takes charge following the dismissal of President Viktor Yanukovych on Saturday. Turchynov told lawmakers they had until Tuesday to form a new unity government.
Parliament also voted to seize Yanukovych’s luxury estate near Kiev, which protesters entered on Saturday.
The whereabouts of Yanukovych, who described parliament’s decision to vote him out as a coup, remain unclear.
Thousands of opposition supporters remain in Independence Square, where the atmosphere is described as calm.
Her release was one of the conditions of the EU-Ukraine trade pact that President Yanukovych rejected last year – triggering the protests that led to the current crisis.
The health ministry says 88 people, mostly protesters, are now known to have been killed since 18 February.
Turchynov, a close associate of Tymoshenko, described forming a unity government as a “priority task”.
Speaking to the BBC, he also suggested a bid for the presidency in elections scheduled for May 25.
“I want to make Ukraine a modern European country,” he said. “If I can do that through the president’s position, I will do my best.”
In other decisions on Saturday:
Foreign Minister Leonid Kozhara and Education Minister Dmytro Tabachnyk were dismissed.
Media reports have quoted Ukrainian officials as saying he was stopped by border police while attempting to fly to Russia aboard a private plane.
Lawmakers from Yanukovych’s Party of Regions now appear to be disowning him.
“Ukraine was betrayed and people were set against each other. Full responsibility for this rests with Yanukovych and his entourage,” its lawmaker said in a statement carried by Interfax-Ukraine.