Yulia Tymoshenko says she will not be afraid to go to the courts
Tension is rising between candidates in Sunday's presidential election in Ukraine, with Yulia Tymoshenko accusing her main rival of preparing huge fraud.
Mrs Tymoshenko, the prime minister, said a "deliberate disruption of the election process" was taking place on behalf of rival Viktor Yanukovych.
Mr Yanukovych's official victory in the 2004 election was later ruled to be fraudulent and overturned.
He dismissed the claims, saying only the ruling party could arrange fraud.
Mrs Tymoshenko alleged that Mr Yanukovych's Party of the Regions was organising mass fraud in the east of the country, his main power base.
In the Donetsk region, 11% of citizens had applied to vote from home, without any proof that they were not well enough to attend polling stations, she said.
"Such monstrous falsification didn't even happen in 2004," she said.
She has already threatened to resort to the courts if there is evidence of fraud taking place.
But Mr Yanukovych shrugged off the accusations.
"How can the opposition falsify results? Only the authorities have that ability - they have the mechanism, structure, the interior ministry," he said.
Mrs Tymoshenko was one of the leaders of the Orange Revolution that rose up after Mr Yanukovych's false victory last time.
But she has since fallen out with her Orange ally, Viktor Yushchenko, and is hoping to replace him as president.
Mr Yushchenko is running again, though only scores single figures in opinion polls and is given very little chance of re-election.
One agency says Sergei Tigipko has made a surge in the polls
A Russian opinion poll on Wednesday showed an apparent surge by an alternative candidate, Sergei Tigipko, a former economy minister.
Most observers have predicted that Mrs Tymoshenko and Mr Yanukovych will go forward to a second round of voting.
But the latest poll showed Mr Tigipko, on 14.4%, pushing Mrs Tymoshenko, with 13.9%, into third place. Mr Yanukovych was far ahead with 30.5%.
The poll, carried out by a Russian state-run polling agency, VTsIOM, differs from recent Ukrainian recent polls, which have now ceased for the election period.
Both Mrs Tymoshenko and Mr Yanukovych have been accused of having close links with Russia.