Georgia's opposition is calling for mass street protests after accusing the authorities of trying to rig Saturday's snap presidential elections.
Mr Gachechiladze is urging his supporters to rally in Tbilisi
Exit polls suggested President Mikhail Saakashvili won the elections, possibly by enough votes to avoid a run-off.
But opposition leader Levan Gachechiladze said the exit polls had been "falsified".
The vote was seen as a democratic test for the ex-Soviet republic after recent opposition protests were suppressed.
Mr Gachechiladze, the leader of a nine-party opposition coalition that pollsters said had won 28% of the vote, accused the government of trying to rig the election.
"Saakashvili is lying," Mr Gachechiladze told reporters in the capital, Tbilisi.
"The exit polls have been falsified," he said, adding that his team was recording numerous violations around the country.
A senior US State Department official said indications were that Mr Saakashsvili may have won the presidential race, but it was not clear if he had avoided a run-off by winning more than 50% of the vote.
Matthew Bryza, Deputy Assistant Secretary of State for European and Eurasian affairs, called for calm from protestors until the election result could be clarified and assessed by international observers to determine if it was "free and fair".
"It is the rule of law that matters, not the rule of the mob," he said.
Mr Gachechiladze has called on his supporters to celebrate what he said was his victory at a rally in Tbilisi on Sunday.
President Saakashvili, however, said he was confident of victory.
"According to the exit polls and all the data we have won, although as a democratic party we should wait for the central election commission's final results," he told his jubilant supporters.
President Saakashvili is hoping to avoid a run-off
The BBC's correspondent says there has been a mood of celebration around Mr Saakashvili's campaign headquarters, with people waving flags and cars tooting their horns.
No official results have yet been published, and analysts are urging caution, as 20% of respondents in the exit poll refused to say who they had voted for.
The authorities have denied allegations of fraud, and acting President Nino Burjanadze told the BBC that the elections had been "free, fair and democratic".
But our correspondent says that after a bitter and divisive election the dispute seems certain to continue.
Mr Saakashvili called the early election in an attempt to prove his democratic credentials after huge opposition protests were suppressed in November.
Along with the presidential election, Georgians were asked to vote on whether they should have a parliamentary election in the coming months, and whether the country should join Nato.
Hundreds of foreign observers have been monitoring the ballot.
Mr Saakashvili, a US-educated lawyer, came to power after street protests in 2003, dubbed the Rose Revolution.
His 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.