Yulia Tymoshenko, leader of the Batkivschyna party and a presidential candidate, was interviewed at the Ukrainian Interior Ministry on Tuesday as a witness in two criminal cases dealing with violations of election regulations, the Batkivschyna party said in a statement.

Tymoshenko said the National Police have opened two criminal cases based on her reports, one into the bribing of voters in incumbent President Petro Poroshenko's favor and the other into manipulating the voter register.

"Our goal is to fully stop bribery and put the elections back onto a fair democratic path. And we will accomplish this. Poroshenko won't be able to commit a nationwide act of bribery," Tymoshenko said following the interview at the Interior Ministry.

Tymoshenko said her team has gathered abundant evidence proving that the incumbent president is buying up votes through a "pyramid of bribery " led by Serhiy Berezenko, a parliamentarian from the Petro Poroshenko Solidarity Bloc.

The people campaigning for Poroshenko want to give UAH 1,000 each to six million Ukrainians, she said.

"They believe six million people will secure a second round for him," she said.

"We have recorded the de facto distribution of questionnaires and conclusion of agreements for campaigning services, which is prohibited by law. Each person signing a questionnaire and an agreement for campaigning services receives UAH 1,000, the so-called Petro's Thousand. The whole country already knows who is bribing the people and how much the president will give them for each vote," Tymoshenko said.

Another vote-rigging method used by Poroshenko's team is inflation of the voter register, she said.

"While the population is shrinking and Ukrainians are leaving for other countries in droves, the CEC has decided to extend the voter register by more than one million people. It's clear that Poroshenko wants to take advantage of ghost voters and cast ballots for them," Tymoshenko said.

Tymoshenko said she was convinced that her team across Ukraine will oppose the president's attempts to bribe voters by collecting evidence and passing it to the Interior Ministry, and that police would react in a timely fashion to attempts by Poroshenko's associates to rig the choice made by Ukrainians.

"We hope [Interior] Minister [Arsen] Avakov, just as he has said, won't allow a massive buy-up of votes and will make public the facts, including those provided by our team," she said.