Saakashvili's popularity has plummeted since last August's disastrous war with Russia, and his failure to carry out democratic reforms promised after the 2003 "Rose Revolution" that brought him to power.
The opposition Democratic Movement-United Georgia party, led by Saakashvili's former ally Nino Burjanadze, said on Thursday that 60 of its activists have been arrested in the town of Rustavi, to the southeast of the capital during the night. Georgia's Interior Ministry has denied the report.
Six hundred people have gathered outside parliament, and several thousand more are heading towards the building, from the direction of Tbilisi University, a RIA Novosti correspondent reported. The protest is set to begin at 2:00 p.m. local time (10:00 GMT).
The opposition says that at least 150,000 people will participate in the rally, timed to coincide with the 20th anniversary of a Soviet crackdown on Georgian independence protests. Opposition leaders earlier said the protests would continue until Saakashvili resigns and calls early elections.
Hundreds of riot police have gathered in front of the parliament building and the presidential residence in central Tbilisi. Fire crews are also at the ready, in apparent anticipation of arson attacks similar to those seen during this week's anti-government riots in Moldova.
"These are routine security measures taken during mass demonstrations," Interior Ministry spokesman Shota Khizanishvili said, adding that police would also be inside buildings close to the site.
Analysts fear the protests could escalate into violent riots similar to those in 2007, which were brutally dispersed with police firing at protesters with rubber bullets and tear gas.
Analysts say protesters are unlikely to achieve their goal of forcing Saakashvili from power, due to the lack of a strong alternative leadership.
The U.S.-educated president, who has refused to step down, tried on Thursday to unite the government and the opposition against what he sees as their common enemy, Russia.
"We have a common goal... we seek the final liberation of our Georgia, and its development into a modern, democratic, European state," he told a crowd at the memorial to victims of the 1989 Soviet crackdown in Tbilisi.
