Oleksandr Turchynov takes charge following the dismissal of President Viktor Yanukovych on Saturday. He told lawmakers they had until Tuesday to form a new unity government.
Former PM Yulia Tymoshenko, who was freed from jail on Saturday, has ruled out becoming prime minister again.
Her release was one of the conditions of the EU-Ukraine trade pact that Viktor Yanukovych rejected last year.
The move triggered the protests that led to the current crisis.
The health ministry says 88 people, mostly protesters, are now known to have been killed in clashes since February 18.
Thousands of opposition supporters remain in Kiev’s Independence Square, heeding opposition calls not to disperse.
In response to reports that her name was being mentioned as a possible candidate, Yulia Tymoshenko issued a statement reading: “No-one has agreed or discussed this with me.
“Thank you for your respect but I would like you not to consider my nomination for the post of the head of government.”
Oleksandr Turchynov, a close associate of Yulia Tymoshenko, described forming a unity government as a “priority task”.
“We don’t have much time,” one of the opposition leaders, former world champion boxer Vitaly Klitschko, said as parliament began its debate.
“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 an address on Saturday, aired before lawmakers voted to remove him, Viktor Yanukovych refused to officially stand down. He is last thought to have been in Kharkiv after travelling there late on Friday night.
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