Oleksandr Turchynov takes charge following the dismissal of President Viktor Yanukovych on Saturday.
Mr Turchynov told MPs they had until Tuesday to form a new unity government.
Parliament also voted to seize Mr Yanukovych's luxury estate near Kiev, which protesters entered on Saturday.
The whereabouts of Mr 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.
Late on Saturday, former Prime Minister Yulia Tymoshenko, freed from detention in the eastern city of Kharkiv after a vote in parliament, urged opposition supporters in Independence Square to continue protesting.
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.
Mr Turchynov, a close associate of Ms 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.
Speaking to the BBC, he also suggested a bid for the presidency in elections scheduled for 25 May.
"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.
Arrest warrants were issued for former Incomes Minister Oleksandr Klimenko and former Prosecutor-General Viktor Pshonka.
Parliament lowered the official status of the Russian language by cancelling a law brought in by Mr Yanukovych.
Mr Yanukovych refused to officially stand down.
He is last thought to have been in Kharkiv after travelling there late on Friday night.
Media reports have quoted Ukrainian officials as saying he was stopped by border police while attempting to fly to Russia aboard a private plane.
MPs from Mr 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 MPs said in a statement carried by Interfax-Ukraine.
US Treasury Secretary Jack Lew discussed Ukraine with Russian Finance Minister Anton Siluanov on the sidelines of the G20 meeting in Sydney, a US official said.
Mr Lew told reporters the US was ready to work "with other countries including Russia" to help Ukraine "as it implements reforms to restore economic stability and seeks to return to a path of democracy and growth".
However, Mr Siluanov has left open the question of whether Russia will pay the next instalment of financial help for Ukraine, worth $2bn.
"We are planning to wait until a new government is formed and until we understand the policy of this government and then we shall make a decision," Siluanov was quoted by website Gazeta.ru as saying.
In a telephone conversation with Russian Foreign Minister Sergei Lavrov, US Secretary of State John Kerry noted the "peaceful atmosphere" prevailing in Kiev after the departure of President Yanukovych, officials said.
Russia and the US have been on opposite sides during the Ukraine crisis, which the US, along with the EU, backing the opposition.
The European Union, too, has said it stands ready to assist a new government.
Economic and Monetary Affairs Commissioner Olli Rehn said in Sydney: "It is important that we provide a clear European perspective for the Ukrainian people."
UK Foreign Secretary William Hague told the BBC he would be speaking to Mr Lavrov on Monday.
"It's very important for us to persuade Russia that this need not be a zero-sum game," he said.
Acting President Oleksandr Turchynov
- Born in Dnipropetrovsk, eastern Ukraine, March 1964
- Trained as metallurgist and economist
- 1980s - Local communist Komsomol youth leader
- 1993 - Economic adviser to ex-President Leonid Kuchma
- 1994 - Co-founded centre-left Hromada party with Pavlo Lazarenko
- 1998-2007 - Elected to parliament
- 199 - Deputy leader of Yulia Tymoshenko's Fatherland party
- 2004 - Campaigner in Orange Revolution
- 2005 - Head of Ukraine Security Service (SBU)
- 2007 - 2010 - Deputy PM February
- 2014 - Parliament speaker, then acting president
Source: BBC News