Georgian President Eduard Shevardnadze has declared a state of emergency after the opposition seized the parliament building and his presidential offices.
Opposition supporters hailed a "velvet revolution"
The move came after weeks of protests over disputed elections won by Mr Shevardnadze and his political allies.
But the president says he will not resign over the "attempted coup" and has warned that the army will take over if the emergency law is not endorsed.
The outgoing parliament speaker says she has become interim president.
Adding to the political confusion, Nino Burjanadze, said she had taken over until Mr Shevardnadze's resignation was "resolved".
It is not known where the Georgian president is staying.
Russian Foreign Minister, Igor Ivanov, has now arrived in Georgia to mediate in the turmoil.
Saturday's dramatic developments began when thousands of people led by main opposition leader Mikhail Saakashvili stormed the parliament building in the capital Tbilisi.
Mr Shevardnadze had been addressing the first session of the newly-elected parliament after the victory of his party and its allies in the 2 November elections, which were declared fraudulent by international observers.
EVENTS LEADING TO 'COUP'
2 Nov: Parliamentary elections held
4 Nov: Anti-Shevardnadze protests begin
6 Nov: Election officials announce two-week delay until final results
18 Nov: Thousands of Shevardnadze supporters rally in Tbilisi
20 Nov: Central Election Commission declares Shevardnadze's party election victor; US claims "massive vote fraud"
21 Nov: Opposition supporters descend on Tbilisi
22 Nov: Parliament stormed; Shevardnadze denounces 'coup' and declared state of emergency
The president was bundled away by armed bodyguards as protesters entered the building.
The opposition supporters then drove Mr Shevardnadze's supporters from parliament under a hail of books and pens after a number of fist-fights.
Live television pictures showed scenes reminiscent of the revolutions that toppled the communist governments of Eastern Europe more than a decade ago.
Mr Saakashvili and other opposition leaders declared they had carried out a bloodless - or "velvet" - revolution.
Mr Shevardnadze announced the state of emergency hours after being forced out of parliament.
"With such people around, a civil war may start tomorrow," he said.
"If I show weakness now, people will no longer forgive me."
Under Georgian law, Mr Shevardnadze must obtain parliamentary approval for the state of emergency within 48 hours - but with such chaotic scenes inside parliament, this appears unlikely.
The country's armed forces have so far not intervened in events, and it is unclear whether they would enforce the state of emergency called by Mr Shevardnadze.
Armed personnel carriers have been spotted on Tbilisi's streets, but security forces allowed demonstrators into parliament - some even offering words of encouragement, says BBC correspondent Chloe Arnold.
They later made no attempt to stop protesters storming the presidential residence.
Mr Shevardnadze himself has not been seen in recent hours, though one of his aides reportedly said he was at one of his country residences.
Georgian television says opposition leaders have been invited to a meeting on Sunday with Mr Ivanov, the Russian foreign minister - and "have not rejected the offer".
Arriving in Tbilisi, Mr Ivanov denied that Russia was attempting to interfere in Georgian affairs, but said Russia was "not indifferent to its fate".
Governments around the world have urged both sides not to allow the situation to become violent.
The United States has called for dialogue "with a view to restoring calm and reaching a compromise solution acceptable to all and in the interest of Georgia".
And the former Soviet republics condemned the opposition move.