The Organisation for Security and Co-operation in Europe (OSCE), Europe's main election monitor, said on Sunday that the previous day's snap election was "competitive" and "valid".
"While the ... election in Georgia was in essence consistent with most international standards for democratic elections, significant challenges were revealed," an OSCE statement said.
"I perceive this election as a viable expression of the free choice of the Georgian people" Alcee Hastings, a senior OSCE election observer, said in the statement.
Russia's foreign ministry dismissed the endorsement of the election as "superficial".
But Matthew Bryza, Washington's top emissary to the region, the deputy assistant secretary of state, said 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," he said.
Despite the Western reactions, up to 10,000 opposition supporters gathered in the snow in Tbilisi's central Rike Square to hear leaders accuse the authorities of stealing Saturday's election.
"We face terror," Gachechiladze said at the rally.
|"Mikhail Saakashvili: it is impossible to defeat the Georgian people. We will defend our vote by legal means"|
Levan Gachechiladze, opposition presidential candidate
"Mikhail Saakashvili: it is impossible to defeat the Georgian people. We will defend our vote by legal means."
Gachechiladze claimed that he came first in the vote and said a second round was necessary.
He cited a tally by his supporters who served on election commissions across the country.
Gachechiladze said: "Saakashvili lost and it cannot happen that Georgia will not defend it's freedom, that we won't win."
He said more rallies would take place after the Orthodox Christmas on Monday.
The protest on Sunday passed off peacefully.
Late on Saturday, Saakashvili's supporters poured onto the streets, tooting car horns and waving white-and-red national flags, as exit polls showed that he was likely to win results.
He called for reconciliation in a speech to supporters at a celebratory concert.
"If the final results confirm that I have won in the first round, then I will assume the honour and responsibility to serve all of Georgia for the next five years," Saakashvili said.
"I'm extending my hand to those who voted for me and to those who took part in the elections."
Gachechiladze said the results were "falsified", and called on "all of Georgia to come to make sure we don't lose our country".