The announcement made it clear that after two and a half years in prison she intends to play an active role as Ukraine struggles with political upheaval, a severe financial crisis and menacing moves by Russia, which annexed the Crimean Peninsula a week ago, has massed troops along the eastern border and poses an enormous threat to this country’s gas supply.
Ms. Tymoshenko announced her political intentions on a day of fast-moving diplomatic and economic developments in the Ukraine crisis, the focal point of escalating tensions between Russia and the West.
The International Monetary Fund announced an agreement to provide Ukraine with up to $18 billion in urgently needed loans; the House and Senate in Washington approved a $1 billion aid package and new sanctions against Russians and Ukrainians deemed responsible for the Crimea annexation; and the United Nations General Assembly voted overwhelmingly to reject that annexation, a symbolic step that Western nations called a rebuke to Russia that showed its isolation.
And in a possible sign of further internal turmoil, hundreds of members of Right Sector, the extreme nationalist group that played an important role in the convulsions that culminated in a change of government last month, gathered outside Parliament in Kiev on Thursday night, demanding the resignation of the interim interior minister.
They were angry over the killing by police officers earlier in the week of a far-right activist, Oleksandr Muzychko, whom the Interior Ministry had described as a violent member of an armed criminal group.
The crowd later dispersed but promised to return Friday.
Ms. Tymoshenko, a charismatic but also potentially polarizing figure, ran unsuccessfully for president in 2010 against her archrival, Viktor F. Yanukovych, who was toppled in the February upheaval and fled to Russia.
She is now the best-known candidate in a field that includes Petro Poroshenko, a billionaire known as the chocolate king; Vitali V. Klitschko, a former boxing champion who is a leader in Parliament; and Sergey Tigipko, a veteran lawmaker and former vice prime minister under Mr. Yanukovych who announced his candidacy on Thursday as well, calling himself an independent.
Ms. Tymoshenko was jailed on charges that were criticized by the West as politically motivated and as stemming from her rivalry with Mr. Yanukovych.
She was freed from a prison hospital just hours after Mr. Yanukovych departed the presidential residence last month, and she immediately went to Independence Square, the gathering point for demonstrators in Kiev, where she received a mixed reception.
Although Ms. Tymoshenko has long harbored ambitions to be president, and despite her candidacy’s being widely expected, she had seemed to waver a bit in recent weeks.
While the demonstrators in Kiev were thrilled about Mr. Yanukovych’s ouster, many have expressed suspicions of anyone with longstanding ties to Ukrainian politics, which has a history of corruption and mismanagement.
Ms. Tymoshenko, 53, held a news conference to announce her ambitions at the headquarters of her political party, Fatherland, in Kiev.
“I plan to run for the position of president of Ukraine,” Ms. Tymoshenko said.
She also said that she considered President Vladimir V. Putin of Russia, who moved to annex the Crimean Peninsula in the aftermath of Mr. Yanukovych’s departure and has deployed at least 20,000 Russian troops along the Russia-Ukraine border, to be “enemy No. 1 of Ukraine.”
She emphasized her experience, arguing that she was more qualified than any rival.
Sitting with crutches on either side of her, the legacy of chronic back problems aggravated by her incarceration, she said Ukrainian lawmakers “can’t imagine what I, as a politician, have experienced for myself in prison.”
Ms. Tymoshenko, who is known for her trademark blond braid, generated new controversy this week after she was heard in a recorded telephone conversation using expletives and a derogatory term for Russians, and saying that Russia should be destroyed for its invasion and annexation of Crimea.
“I am hoping that I will use all of my connections and will get the whole world to rise up so that not even scorched earth would be left of Russia,” Ms. Tymoshenko said in the call, posted on YouTube.
Similar calls, believed to have been recorded by Russian intelligence services, have been posted on YouTube, including one between an American assistant secretary of state, Victoria Nuland, and the American ambassador in Kiev, Geoffrey R. Pyatt.
Ms. Tymoshenko, writing on Twitter, suggested that some of the conversation had been altered but appeared to confirm the general contents, and apologized for using expletives.
Since Mr. Yanukovych’s ouster, which the Kremlin has denounced as a coup supported by the West, the Fatherland party has held a strong grip on the new provisional government.
Both the acting president, Oleksandr V. Turchynov, and the acting prime minister, Arseniy P. Yatsenyuk, are leading members of Fatherland.
The party voted at a conference last year to nominate Ms. Tymoshenko as its candidate for president should she be able to run.
The I.M.F. loan agreement, announced in Kiev, will hinge on the country’s steps to let the value of its currency float downward, to cut corruption and red tape, and, crucially, to reduce huge state subsidies for the consumption of natural gas.
The energy subsidies alone represent roughly 8 percent of Ukraine’s gross domestic product, and Russia has said that it intends to raise the price of natural gas to Ukraine on April 1.
Ukraine is largely dependent on Russian supplies and already owes the Russian energy company Gazprom more than $1 billion.
The deal, subject to the approval of the monetary fund’s board next month, is intended to get the new government through looming debt obligations when its hard-currency accounts have been sharply diminished by the months of unrest.
The two-year loan package, the fund said in a statement, is expected to unlock more loans, including from the United States and the European Union, which should bring the total over two years to $27 billion.
The loans will be more spread out and less onerous than the $15 billion Russia had promised Mr. Yanukovych before he fled the country.
Mr. Yatsenyuk told Parliament on Thursday that the country was “on the brink of economic and financial bankruptcy” and required urgent steps in conjunction with the monetary fund.
He announced legislation to prevent “financial disaster,” including provisions that would freeze the minimum wage and raise taxes on the largest businesses.
The White House press secretary, Jay Carney, praised the I.M.F. announcement, saying it was “a powerful sign of support from the international community for the Ukrainian government.”
Source: The New York Times