War in August, when Russia crushed a Georgian assault on breakaway South Ossetia and sent tanks to within 40 km (25 miles) of Tbilisi, emboldened critics who argue the president has made too many mistakes to stay in power until 2013.
Opposition leaders, their ranks swollen by defectors, are predicting turnout of 150,000, and say protests will continue daily until Saakashvili, 41, resigns and calls elections.
Fire crews and hundreds of police in full riot gear entered the courtyard of the parliament in central Tbilisi overnight.
"We have set ourselves the goal of removing the incumbent illegitimate president from power and we will achieve this goal," opposition leader Salome Zurabishvili told reporters. "The rally will not disperse until Saakashvili resigns."
Analysts warn frustrations risk boiling over into unrest. They say they doubt the opposition's unity, its strength of leadership and the level of support it commands beyond Tbilisi.
ANNIVERSARY OF SOVIET CRACKDOWN
Many Georgians are tired of political bickering in the capital and are sympathetic to government calls for stability amid the global crisis, which the International Monetary Fund warns will have deeper impact in Georgia than first thought.
Some Georgians see Saakashvili as brash and impulsive, and question his handling of the war. But he draws support from the prevailing consensus in the country that Russia was to blame.
The West -- drawing oil and gas through Georgia from the Caspian Sea -- is watching for a possible repeat of a November 2007 crackdown, when police firing teargas and rubber bullets dispersed the last major demonstrations against Saakashvili.
Diplomats warn such scenes would be disastrous for Saakashvili's standing abroad.
His young, mainly Western-educated team came to power blessed by former U.S. President George W. Bush as a "beacon of liberty," but the light has faded and diplomats say Barack Obama's administration will be less forgiving.
Analysts warn opposition anticipation has been pumped so high that leaders might struggle to control hardliners.
The fear has been fed by the authorities, who last month said they had uncovered a plot to overthrow the government. Police arrested 10 men with suspected opposition links and released secretly filmed video of them apparently buying weapons.
The government says it suspects the hand of Russia. The opposition dismisses the allegations as a smear campaign.
Thursday's rally is timed to coincide with the 20th anniversary of a deadly crackdown by Soviet troops in the twilight of the Soviet Union against Georgian protesters demanding independence for the republic of 4.5 million people.
Hundreds of Georgians laid red roses at the steps of the parliament on early on Thursday to remember the victims.