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Ukrainian President Petro Poroshenko says his two top election rivals are backed by a self-exiled tycoon who wants revenge for the nationalization of his bank.

Poroshenko said at a rally in the city of Lviv on Thursday that billionaire businessman Ihor Kolomoyskyi is trying to unseat him by supporting comedian Volodymyr Zelenskiy and former Prime Minister Yulia Tymoshenko in Sunday's election.

Opinion polls have shown Zelenskiy leading the race. He and Tymoshenko have rejected claims that Kolomoyskyi is the funder and mastermind of their campaigns.

Poroshenko blamed the ex-prime minister for Ukraine's past economic woes and alleged that a Zelenskiy victory would help Russia.

Russia annexed Ukraine's Crimea in 2014 in a move that most nations see as illegal and also backed a separatist rebellion in eastern Ukraine.