Speaking at the rally, opposition leader Levan Gachechiladze called for the removal of the government, and warned that he and his allies would not allow authorities to steal a parliamentary vote scheduled for May.
"Today a new November begins in Georgia," Gachechiladze said, referring to demonstrations that paralyzed the capital during that month in 2007, and led to a state of emergency. "This is going to be a permanent rally and it will last until we reach our goal. And this goal is elections, free elections."
The demonstrators, who gathered in front of the parliament building, denounced government control over media and demanded fair access to television and accused Saakashvili of stealing the January 5 presidential election by fraud.
Official results gave Saakashvili 53 percent of the vote in the presidential elections, averting a runoff against his nearest rival, Gachechiladze, who won 25 percent.
Observers from the Organization for Security and Cooperation in Europe (OSCE) have called the vote a "triumphant step" for democracy in Georgia, but nevertheless pointed to some irregularities.
Zviad Dzidziguri, the leader of the Conservative Party, and four other opposition leaders announced today that they were going on a hunger strike to press their demand for a runoff between Saakashvili and Gachechiladze.
Gachechiladze said the opposition will hold another rally near parliament March 11 to protest a government-backed bill revising the assembly's structure. He denounced the bill as an attempt by Saakashvili to preserve his control over parliament.
On March 8, Saakashvili urged the opposition to refrain from protesting amid heightening tensions with Russia.
Tbilisi officials have decried as "immoral and dangerous" Russia's decision on March 6 to withdraw from a CIS treaty imposing sanctions on Georgia's breakaway region of Abkhazia.
Saakashvili was elected to the presidency in 2004 after the Rose Revolution thrust him into power. But in November 2007, he cut short his five-year term and ordered an early presidential for January after police violently dispersed antigovernment protesters. The president also imposed a state of emergency at the time.