Georgia's opposition said it would launch more protests and block streets in the capital Tblisi, after President Mikheil Saakashvili ignored calls to resign.
Some protesters chanted: 'Misha, get out!'
"Because Saakashvili has not decided to resign, the opposition has decided to start a national disobedience campaign," Kakha Kukava, a co-leader of the opposition Conservative Party, told about 25,000 protesters outside parliament.
"As of 6 p.m. this Friday the protesters will block main streets throughout Tbilisi," Kukava said.
Opposition leader David Gamkrelidze warned of possible violence were Saakashvili to continue to dismiss the protests. "If the government keeps ignoring the demonstrations, this could lead to a people's revolution," he said in an interview on the Maestro television channel. The marches will go on until Saakashvili quits, opposition officials said.
Saakashvili has refused to step down despite tens of thousands of protestors nationwide demanding he leave his job. The president has instead offered to talk with the opposition to resolve the growing crisis.
"The government is ready for a dialogue with all political powers, moderate or radical," Saakashvili said in a national television address.
Some 80,000 to 150,000 people marched in Tblisi
Georgia's opposition launched mass demonstrations against Saakashvili on Thursday. It accuses the president of leading the Caucasus nation into a disastrous war against Russia in August, and of using police and national security forces to repress domestic dissent.
Saakashvili has from time to time offered to discuss Georgia's difficult economic and political situation with his critics, but so far the opposition has rejected the offers as insincere.
President accused of undermining Georgia
Saakashvili's fall in popularity, still high less than one year ago, accelerated after last year's war with Russia led to the total loss of the Georgian provinces South Ossetia and Abkhazia.
Both states subsequently declared independence. Moscow has recognized South Ossetia and Abkhazia as independent states, and maintains combat troops in both locations.
"Saakashvili has undermined the reputation of Georgia, and he has divided our land," said Levan Gachechiladze, a former presidential candidate.
Now an opposition leader, Gachechiladze estimated the size of the anti-Saakashvili crowd in Tbilisi at 150,000 participants. Independent observers put the size of the demonstration at some 80,000.
Tens of thousands of anti-Saakashvili protestors took to the streets in Batumi, Poti, and other Georgian cities, according to Georgian news reports.
"Misha, get out!" chanted the crowd at some locations.
Saakashvili takes protests in stride
Saakashvili has appeared untroubled by the protests
Saakashvili's public reaction to the demonstrations thus far has been mild, with the Georgian leader telling reporters he viewed the marches as proof of the country's support freedom of speech and expression.
The Saakashvili administration in November 2007 unleashed police on similar anti-government demonstrations, citing the marches' alleged threat to public order, and violation of public disturbance law.
Some Saakashvili critics argued the 2007 police assault on generally peaceful protestors recalled a Soviet repression of a Georgian nationalist demonstration in April 1989 killing 20 marchers.
Saakashvili came to power in 2003 after mass demonstrations in Tbilisi forced then-president Eduard Shevardnadze from office.
Georgia's opposition in recent months has attempted to pressure Saakashvili into quitting his post, arguing the August war against Russia, and a worsening economy have made his presidency ineffective.
But Georgia's opposition is divided, lacking a true leader or a political agenda beyond kicking Saakashvili out of office.