Reports said around 100,000 to 120,000 people took part in the rally, including members and supporters of opposition parties, representatives of public organizations, college students, and the unemployed.
Opposition leaders have promised to demonstrate daily outside the parliament building until Saakashvili quits, accusing him of rigging elections and authoritarian tendencies.
Former presidential challenger Levan Gachechiladze said the demonstrators "have no other way out but to stand here" until Saakashvili resigns.
"Mikhail Saakashvili should realize that it is high time for him to resign. Most Georgian citizens demand his resignation," said Nino Burdzhanadze, the former parliament's speaker, before the rally.
The protesters raised clenched fists as they chanted slogans, urging the president to step down immediately.
The most bitter criticism was directed at the president's handling of the August conflict last year.
Opposition groups said Saakashvili made a serious mistake when he launched an assault on the breakaway region of South Ossetia last year.
