Outgoing President Viktor Yushchenko has signed into law a change in Ukraine's electoral legislation three days before the vote on his successor.
The amendment allows vote-counting at polling stations across Ukraine to go ahead whether or not representatives of both candidates are present.
PM Yulia Tymoshenko said the change would help supporters of her opponent, Viktor Yanukovych, to commit fraud.
Mr Yanukovych said his rival's complaints were a sign of weakness.
Sunday's vote is a run-off, between the winner and runner-up of the first round on 17 January. President Yushchenko was ejected in the first round.
Mrs Tymoshenko has called on her supporters to take to the streets if she is defeated in Sunday's poll, saying the protests could be larger than those of 2004's Orange Revolution, which swept Mr Yushchenko to power.
"If we are unable to guarantee the honest expression of the people's will and honest results, we will mobilise the people," said Mrs Tymoshenko in Kiev on Thursday.
"I ask you not to allow Yanukovych to rape our democracy, our election and our country!"
But former Prime Minister Mr Yanukovych, who finished 10% ahead of his rival in a first round of voting last month, said her threat was merely an act of desperation.
"This is a sign of her weakness and an indication that she is losing," he said during a penultimate day's campaigning in central Ukraine.
"If people go (to the protests), it will just be a handful, lovers of the same kind of meals that Tymoshenko loves to cook - filth, lies and slander."