The ten-thousand protest was announced on Georgian TV Saturday by Levan Gachechiladze, who officially falls some 27% behind Saakashvili, but claims the vote was rigged.
One of Gachechiladze supporters said addressing the gathering that he had evidence proving that Gachechiladze had won the election.
The protest was peaceful and controlled by the police. Gachechiladze told his supporters there would be no protests on Monday, since Georgia would be celebrating Orthodox Christmas, but called for another rally on January 8 saying his supporters would lodge violation complaints to the Georgian Central Election Commission (CEC).
With the official results available from 603 polling stations out of 3,512, U.S.-educated Saakashvili has gained 51.64% of the vote, and Gachechiladze has won 24.56%.
The election commission said the voter turnout was 56.17%, with 1,912,900 voters casting their ballots.
The U.S. and the Organization for Security and Cooperation in Europe said Saturday's snap election in Georgia, which was called by Saakashvili in November 2007 after opposition protests were crushed, was broadly fair.
Gachechiladze has said the election was held with gross violations. "We will submit complaints on all violations during elections to the CEC," he said.
Other opposition candidates also questioned the fairness of the election.
Russia's Foreign Ministry also expressed doubt Sunday that the presidential election in Georgia was democratic saying it was accompanied by pressure on opposition candidates and adding that "administrative resource" was widely used.