Gachechiladze told opposition supporters who gathered in the snow in Tbilisi's central Rike Square that he came first in the vote and said a second round was necessary.
Gachechiladze cited a tally by his supporters who served on election commissions across the country.
"Mikhail Saakashvili: it is impossible to defeat the Georgian people. We will defend our vote by legal means," he said.
"Saakashvili lost and it cannot happen that Georgia will not defend it's freedom, that we won't win."
Protesters chanted "Georgia, Georgia" and held their index fingers up to represent the number one, Gachechiladze's number on the ballot paper.
He said more rallies would take place after the Orthodox Christmas on Monday.
'Competitive and valid'
International monitors disagreed with the opposition saying that the poll had revealed "significant challenges" but was "competitive" and "valid".
|"Saakashvili lost and it cannot happen that Georgia will not defend it's freedom, that we won't win" |
Levan Gachechiladze, opposition candidate
"I perceive this election as a viable expression of the free choice of the Georgian people" Alcee Hastings, a senior election observer for the Organisation for Security and Co-operation in Europe (OSCE), said.
The US said that the opposition should respect the report from Western election observers.
"If the experts determine that the election was not rigged, then there is absolutely no justification, and it would be absolutely undemocratic, to claim otherwise," Matthew Bryza, Washington's top emissary to the region, the deputy assistant secretary of state, said.
Nino Burdzhanadze, the parliament speaker who is serving as acting president during the campaign, conceded there had been some violations.
But said her government welcomed the criticism and would seek to correct the mistakes in future elections.
"What is most important is that, in general, as a whole, the elections were free and fair and democratic," she said.