Georgia's opposition has stepped up its challenge to presidential election results suggesting the re-election of Mikhail Saakashvili.
Unconfirmed results give Mr Gachechiladze 25% of the vote
The opposition disputes the result and has threatened a hunger strike and a massive protest on Sunday if demands for a runoff are not met.
Near complete results give pro-Western Mr Saakashvili more than 50% of the votes - enough to avoid a runoff.
International observers have said the vote was essentially democratic.
Levan Gachechiladze, Mr Saakashvili's main rival, stormed into the offices of the Central Election Commission on Tuesday and accused its chairman, Levan Tarkhnishvili, of fraud.
"You are a falsifier, a swindler. You personally falsified the election, and you will pay for this, I promise you," Mr Gachechiladze said.
"Resign! Otherwise we will protest every day," he said.
Mr Gachechiladze's key ally, Salome Zurabishvili, said that if there was no second round by Sunday "the whole of Georgia will be out on the streets".
Mr Tarkhnishvili said the opposition should make complaints through official channels or the courts.
The opposition has "all possible tools to act within the framework of law," he said.
With around 90% of votes counted, Mr Saakashvili had won 52.1% of votes cast.
Mr Gachechiladze had 24.98%.
Mr Tarkhnishvili has said that an unofficial tally showed that victory for Mr Saakashvili was assured.
On Tuesday, Western ambassadors met Mr Gachechiladze and told the opposition to respect the result of Sunday's vote.
But after the meeting, Mr Gachechiladze pledged to hold the mass demonstration next Sunday and said some of his supporters would go on a hunger strike from Wednesday.
Mr Saakashvili has strengthened ties with Nato and the EU
"They should either kill me or organise a second round of elections," Mr Gachechiladze told reporters.
Opposition leaders claim to have documentary evidence of fraud and have said they will challenge the results in court.
Mr Saakashvili, a US-educated lawyer, swept to power after street protests in 2003, dubbed the "Rose Revolution".
He called the snap election in an attempt to prove his democratic credentials after huge opposition protests were suppressed last November.
Mr Saakashvili's term as president has seen Georgia strengthen its ties with Nato and the European Union.
But relations with Moscow have soured and Georgia's economy has been badly hit by a Russian ban on Georgian goods.