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.
Later, in a TV address, Mr Turchynov suggested Ukraine would re-open talks with the EU about closer links.
Mr Yanukovych's rejection of an EU-Ukraine trade pact triggered the protests that toppled him.
In his address, Mr Turchynov said he was "open to dialogue with Russia" as long as Moscow respected "Ukraine's European choice", according to translated remarks carried by the AFP news agency.
Meanwhile, opposition leader 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 Mr Yanukovych rejected last year.
The opposition-controlled Ukrainian parliament has voted to appoint its freshly-elected Speaker Aleksandr Turchinov as acting president. MPs supporting the move arguing that President Yanukovich has de facto resigned his office.
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Viktor Yanukovich previously rejected opposition calls to voluntarily step down, which would have saved the parliament taking constitutional impeachment procedures. But parliament, which sees itself as the only legitimate body in Ukraine, decided to replace the president without an impeachment.
Turchinov was elected the new speaker of the parliament on Saturday after his predecessor, Vladimir Rybak, resigned his position after the opposition took control of the legislature.
Parliament is considering a number of key decisions that need to be taken in order to assume full power in Ukraine, including appointing a new prime minister, a new prosecutor general and other top officials.
It has also voted to oust top figures of the Yanukovich government, with the latest victims being acting Foreign Minister, Leonid Kozhara, acting Education Minister Dmitry Tabachnik and acting health minister Raisa Bogatyryova.
Among other bills the parliament is to consider are one outlawing Yanukovich’s Party of Regions and the Ukrainian Communist Party, both of which have elected MPs in the parliament. And another one seeks to censor Russian media, accusing them of biased reporting on the protests in Ukraine.
The new authorities in Kiev say they are seeking the arrest of former senior officials, like former Incomes Minister Aleksandr Klimenko and former Prosecutor General Viktor Pshonka. Both were reportedly stopped on Saturday by border control due to lack of proper paperwork, as they were trying to leave Ukraine.
The legitimation of the power grab comes a day after several regions in eastern and southern Ukraine announced that they do not consider the Ukrainian parliament legitimate and would not abide by its orders. The regions also said they would be forming local militias to resist possible incursions of radical activists from western Ukraine and Kiev.
The three month-long political crisis in Ukraine escalated on Tuesday, with radical opposition activists and riot police engaged in two days of gunfighting in Kiev, resulting in some 80 people losing their lives. In the face of the bloodshed, the central Ukrainian government collapsed under opposition pressure, while President Yanukovich left the capital and went to the east of the country.
The ‘Soviet Soldier’ – a monument commemorating the collective sacrifice of the Soviet army against Nazi forces – has been toppled in western Ukraine. This follows the country-wide fall of some two dozen Lenin statues.
The taking down of the ‘Soviet Soldier’ in the town of Stryi, Lvov region, turns a new page in the chaos that gripped the nation in November, and has taken on dangerously nationalist overtones in the past fortnight.
The city administration’s website first reported on the story of the monument, erected in 1965 as a companion piece to two other objects: an obelisk with WWII engravings and the Eternal Flame over the tomb of the Unknown Soldier.
No official confirmation of any orders for its removal has been given.
Oleksander Turchinov, Ukraine's acting president, has said that his country is ready for talks with Russia to try to improve relations, but made clear that Kiev's European integration would be a priority.
In an address to the nation, Turchinov said that Ukraine's new leadership was ready to put Kiev-Moscow relations on a "new, equal and good-neighbourly footing that recognises and takes into account Ukraine's European choice."
"Another priority... is the return to the path of European integration," he added.
He also said the next government would have the task of stabilising the economy that was at risk of default.
Earlier in the day, Ukraine's parliament voted to temporarily hand over the duties of president to Turchinov, the speaker of the assembly, who told deputies to agree on the formation of a national unity government by Tuesday.
The development comes one day after the parliament voted to oust Viktor Yanukovich from presidency, setting May 25 as the date for new presidential elections, and two days after an agreement was reached with Yanukovich on the need to form a national unity government.
Turchinov is a close ally of former Prime Minister Yulia Tymoshenko, the president's main foe.
Parliament also voted to oust the foreign minister and was told by the country's acting prosecutor that an order had been given to detain the former incomes minister and the former prosecutor-general.
The whereabouts of Yanukovich remained unclear on Sunday, a day after he left the capital and rival Tymoshenko was freed from prison, returning Kiev to address a massive, adoring crowd.
The centre of Kiev, meanwhile, was calm on Sunday.
Protesters on Saturday took control of the presidential administration building, and thousands of Ukrainians roamed the suddenly open grounds of the lavish compound outside Kiev where Yanukovich was believed to live.
The political crisis in the nation of 46 million, strategically important for Europe, Russia and the United States, has changed with blinding speed repeatedly in the past week.
Death toll over 80
In the last week, first there were signs that tensions were easing, followed by horrifying violence and then a deal signed under Western pressure that aimed to resolve the conflict that left the unity of the country in question.
The Health Ministry on Saturday said the death toll in clashes between protesters and police had reached 82.
Ukraine is deeply divided between eastern regions that are largely pro-Russian and western areas that widely detest Yanukovich and long for closer ties with the European Union.
Yanukovich's shelving of an agreement with the EU in November set off the wave of protests, but they quickly expanded their grievances to corruption, human rights abuses and calls for Yanukovich's resignation.
This is frustrating, I can find coverage of the Olympics, CNN is talking about a Daytona rainout, MSNBC is covering a story about a Mexican drug lord, and BBC America is showing a Kevin Costner movie.
The Ukraine is freaking lit up, but it's not on any one of the gazillian channels my cable company provides.
For those of you who have been asking for more coverage on this topic, you may want to hold your noses and watch FOX news. They've covered it in good detail, and with a wide range of commentary including not only their usual contributors, but folks like former ambassadors to Ukraine.
Probably the smartest thing the Ukrainian Parliament has done so far, was voting to strip Yanukovych of power. If Putin ever had the notion of sending troops into the country to restore a "democratically elected" president, the Parliament's vote made it a lot tougher.
Of course he still has the excuse that he used when ordering the invasion of Georgia: Russia has the responsibility to protect "Ukranians" of Russian nationality.
One thing seems clear, the next President needs to take care that he doesn't piss off the folks who opposed Yanukovych. They seem to be in a position of believing that violence works.
There's still a lot to unfold.
you may want to hold your noses and watch FOX news.