Thousands have marched on the offices of Georgian President Eduard Shevardnadze to demand his resignation.
Protesters vowed to stay until their demands were met
The rally is said to have been peaceful but riot police blocked the protesters, who formed a human chain around the building.
The demonstration comes after nearly two weeks of protests outside parliament in the capital, Tbilisi, following elections on 2 November.
The opposition, along with foreign observers, said the poll was flawed.
Earlier, Mr Shevardnadze warned that Georgia was on the brink of civil war and urged his opponents to avoid confrontation.
In a television address, Mr Shevardnadze told the nation: "While I am the lawful president of Georgia. I will not allow civil war", adding that the current crisis should be resolved within the framework of the constitution.
Later Mr Shevardnadze spoke by phone to Russian President Vladimir Putin to inform him of the situation.
The conversation came as the Russian Foreign Ministry called for calm in the former Soviet republic, which has had a difficult relationship with Moscow in the last few years.
Aslan Abashidze, the leader of Adzharia region and a powerful ally of Mr Shevardnadze, has been visiting Russia and other countries in the region to drum up support.
Thousands of riot police and interior ministry troops are stationed across Tbilisi.
Protesters stopped in front of troops with riot shields chanting "step down" and "traitor".
Police blocked off roads leading to the chancellery with buses and other vehicles, and special forces troops were seen entering the building with automatic weapons.
Thousands of riot police are stationed across the capital
The protesters formed a human chain without clashing with the troops.
"This is thousands of people who have proved that Georgians will never surrender their right to fair elections and will never resign themselves to what is happening in their country," said opposition leader Zurab Zhvania.
Another opposition leader, Mikhail Saakashvili, called on Georgians to stop paying taxes and hold a nationwide strike until the president stepped down.
He said the president had run out of time after failing to agree to a request to meet the protesters.
"We are within 15 metres of Shevardnadze's offices," he said.
"If he does not have the courage to walk this distance, it will be up
to you to cast your verdict on his criminal regime."
"This man stole everything from us and he is not going to take notice of his own people."
Demonstrators have vowed not to leave until their demands are met, but many were seen dispersing or retreating to the parliament building - where the demonstration began - as the evening wore on.
Some marched to the Central Election Commission, which has been accused of rigging the elections.
Rallies have been held outside parliament all week
Mr Saakashvili told the BBC before the demonstration that he was organising a petition calling for Mr Shevardnadze's resignation.
He said the president had made a promise that he would consider resignation if a million signatures were gathered against him.
However, Mr Shevardnadze reiterated on Friday that he had no intention of stepping down until the presidential election in 2005.
But correspondents say he is deeply unpopular in a country where poverty and corruption are widespread.
There is a wide consensus that Mr Shevardnadze must go, they say, and people are hoping for a scenario similar to the popular uprising which drove former Yugoslav president Slobodan Milosevic from power in 2000.