"We have restarted permanent demonstrations to force the authorities to satisfy our demands," Levan Gachechiladze, who lost to Saakashvili in the January 5 election, said.
"I call on all Georgians to gather and show the authorities the strength of our determination to defend democracy," he said.
Zviad Dzidziguri, an opposition leader, told the crowd that "everything is false in Georgia, false democracy, a false and illegitimate president, a false judiciary, false media freedom.
"We demand true democracy, free and fair elections, free media and an independent judiciary."
Among the protesters, Maya Vashnadze, 46, said "we want to show that Georgia is ready to rejoin the family of democratic European nations."
Kakha Kukava, a member of the opposition Conservative party, announced that opposition parties, which share a broadly pro-Western agenda, would run on a joint ticket at the parliamentary polls expected in May.
The opposition is also demanding the release of people it says were jailed during anti-government protests in November that led to a nine-day state of emergency and the holding of early presidential polls.
Despite the protests, international observers from the Organisation for Security and Co-operation in Europe (OSCE) have given January's poll a clean bill of health.
Friday's demonstration passed off peacefully and those protesting started to leave the rally by late afternoon.
Nino Sturua, an opposition spokeswoman, said the coalition would organise a nationwide hunger strike starting on February 22.