Speaking at a European Business Association meeting in Kiev as the electoral race was heating up, Tymoshenko claimed that President Petro Poroshenko used his position to enrich himself.
Poroshenko's office rejected the allegations as lies.
Tymoshenko also claimed that voters were being offered 1,000 Ukrainian hryvnia (37 US dollars) in exchange for supporting Poroshenko, adding that she asked the Interior Ministry to investigate.
Again, Poroshenko's office quickly dismissed Tymoshenko's claims.
Also Read: 2 convicted over Hungary sludge flood that killed 8
"Tymoshenko has been invariably leading the ratings of liars, and she obviously tries now to strengthen her positions", Poroshenko's press service said in a written statement in response to AP requests for comment.
Ukraine has been hit by economic troubles and a sharp plunge in living standards after Russia's annexation of the Crimean Peninsula and a separatist insurgency in the east.
Tymoshenko served as the country's prime minister in 2007 - 2010.
She later spent two and a half years in prison for signing a gas deal with Russia, which was largely viewed as retribution by her political rival, then-President Viktor Yanukovych.
Also Read: 8 killed in Paris fire: Searches for more victims on
She narrowly lost to Yanukovych in the 2010 presidential vote and to Poroshenko in 2014 after Yanukovych was driven from power by massive protests.
