Tymoshenko's bid for the presidency was overwhelmingly supported by her Fatherland party, whose representatives have key positions in the government installed after Yanukovych's ouster. The two have been rivals for a decade — she became prime minister after Yanukovych's presidential win in 2004 was thrown out due to vote fraud, and he beat her in the 2010 presidential ballot. She was freed when Yanukovych was ousted after spending two years in jail for charges that many in the West viewed as trumped up by his government.
Addressing supporters on Kiev's historic Sofiivska Square, Tymoshenko heavily condemned both Putin and the annexation of Crimea.
"Putin wanted to punish us Ukrainians for our desire for freedom, for our European choice, for our ability to fight," she said. "If you give me your trust to become president, I will not yield another centimeter of Ukrainian land to the aggressors."
Prime Minister Arseniy Yatsenyuk, a close associate of Tymoshenko, vowed that the elections would properly reflect the will of the people.
"As the head of Ukraine's government, I guarantee that we will hold fair and clean elections," he said.
At its conference, the former ruling Party of Regions voted to expel Yanukovych, who has fled to Russia, along with several other senior officials linked with the now-deposed government. The party nominated Mikhail Dobkin, formerly the governor of the heavily Russian-speaking Kharkiv province, as its candidate for president.
More than a dozen other presidential hopefuls are throwing their hat into the ring. One announced his intentions Saturday at a press conference in Kiev dressed as Star Wars character Darth Vader and flanked by supporters in Imperial Stormtrooper and Chewbacca costumes.
A survey in mid-March by the SOCIS polling organization found Poroshenko leading with a wide margin at 36 percent support among likely voters. Klitschko was seen in second place with 13 percent, Tymoshenko had about 12 percent and Dobkin had over 5 percent support.
In Russian-controlled Crimea, meanwhile, the leadership of the peninsula's Tatars announced plans to hold a referendum on declaring autonomy for their community.
Associated Press writers Yuras Karmanau in Kiev and Laura Mills in Crimea contributed to this report.
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