Former Ukrainian prime minister Yulia Tymoshenko is set to run for president in the upcoming May elections.
Speaking at a press conference, Ms Tymoshenko said she would run as a "candidate for Ukrainian unity" and fight against corruption.
Ditching her princess hairstyle, she claimed none of the other candidates understand "the depth of the lawlessness gripping Ukraine" and insisted Russian president Vladimir Putin remains the "number one enemy".
Often regarded as a divisive figure, she promised to find a "common language" between her pro-European supporters and Russian-speakers, many of whom continue to back former President Viktor Yanukovich.
Mr Yanukovich was ousted by the Ukrainian parliament after more than three months of protests in February.
"I will be the candidate of Ukrainian unity," Ms Tymoshenko said. "The west and centre of Ukraine has always voted for me, but I was born in the east, in Dnipropetrovsk."
Earlier this week, Ms Tymoshenko was dragged intro fresh controversy after she appeared to suggest it was time to "take up arms and go wipe out" Russians "together with their leader" in a leaked telephone call broadcast on Russian state television.
She did not name Mr Putin by his name.
Ms Tymoshenko tweeted the conversation had been altered and apologised for using "expletives". She added: "Cheers to the FSB", referring to the Russian security service.
According to opinion polls, chocolate baron Petro Pororoshenko is leading the race for the presidency with the backing of 24.8 per cent of voters, followed by boxer turned politician, Vitali Klitschko with 8.9 per cent of the vote. Ms Tymoshenko is on third place with 8.2 per cent of the vote.
Ms Tymoshenko, 53, was released from prison in February following the ousting of her political nemesis Yanukovich. She was imprisoned in 2011 for abuse of office linked to a gas deal with Russia.
In pictures: Ukraine crisis
In pictures: Ukraine crisis
1/12 Ukraine crisis
People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea.
2/12 Ukraine crisis
In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea.
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Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea.
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Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol.
5/12 Ukraine crisis
Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II.
6/12 Ukraine crisis
A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise.
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An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol,
8/12 Ukraine crisis
9/12 Ukraine crisis
Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th.
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An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th.
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Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya.
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Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.
She rose to the forefront of Ukrainian politics during the 'Orange Revolution' that overthrew Mr Yanukovich's first administration in 2004. Ms Tymoshenko served twice as prime minister and ran for president in 2010, only to be narrowly beaten in a run-off vote by Mr Yanukovich.
The announcement comes just hours after the International Monetary Fund pledged a $14-$18 billion rescue package for Ukraine to help the country's acting government meet its debt obligations against the backdrop of Russian intervention.
The IMF deal is set to unlock support of $27 billion from various countries over the next two years, but it will come with strings attached, including serious austerity measures and higher energy prices.
In a statement, the IMF warned Ukraine is facing "difficult" challenges following the "intense economic and political turbulence of recent months".
In an address to the Ukrainian parliament, interim prime minister Arseniy Yatsenyuk warned Ukraine is teetering "on the brink of the economic and financial bankruptcy" and signalled more pain ahead.
Mr Yatsenyuk said new structural reforms, including raising taxes and a freeze on minimum wage, will hit families hard but he insisted the government had "no choice but the tell the truth".
On Wednesday, Ukraine announced a radical 50 per cent increase in domestic gas prices starting 1 May in an effort to secure the IMF bailout, which made scrapping energy subsidies a key condition for the deal to move forward.
The election is expected to take place on 25 May.