The 42-year-old was elected to parliament last autumn as the head of his pro-Western party Udar ("Punch"). Since then, Klitschko, the reigning WBC heavyweight champion, has campaigned against what he calls authoritarian moves by President Viktor Yanukovych.
He particularly criticszed the jailing of former Ukrainian Prime Minister Yulia Tymoshenko, which was seen as politically motivated by the West.
Klitschko announced his widely expected presidential bid on Thursday in response to efforts by pro-government forces to pass a bill that could prevent him from running because he has permanent residency status in Germany.
He vowed that he "will not be frightened or stopped by this."
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