Georgia's main opposition leader has vowed to "trample" the country's leadership as supporters gathered for protests in the capital Tbilisi.
Opposition supporters have been gathering from outside the capital
Mikhail Saakashvili said a "bloodless revolution" was going on after official results gave the government victory in parliamentary elections.
Amid the tension, the new parliament is set to convene on Saturday.
UN Secretary General Kofi Annan called for "maximum restraint" and urged leaders to seek a political solution.
In a statement, he said he had been following events in Georgia since the 2 November election with "interest and concern".
Vote calls mount
A senior ally of President Eduard Shevardnadze has joined calls for a new vote.
Security Council head Tedo Japaridze said the poll had been marred by fraud.
Both the opposition and international observers have already alleged that the elections were rigged.
Riot police blocked roads around parliament and the president's offices on Friday as thousands of opposition supporters drove or were bussed in from outside the city.
They gathered only streets away from the thousands of supporters of the president.
Opposition MPs have vowed to boycott the new parliament when it meets on Saturday.
Mr Saakashvili called on security forces to stand aside while urging the country's current leaders to flee.
"The Georgian people are here," he said. "We will
trample and run over this regime tomorrow. They had better
flee right now.
"Shevardnadze is finished. His end has come
He said there was a "bloodless, democratic, peaceful, velvet revolution" going on and called on security forces to stand aside.
But Mr Japaridze warned against trying to oust the president.
"No one should be under the illusion that if bloodshed
starts it can be quickly stopped," he said.
The Security Council head, who rarely makes public statements, said he could no longer keep quiet.
Saakashvili asked security forces not to attack protesters
"Today the situation is such that silence ... means moral
guilt in front of one's country and people," he said.
He blamed Mr Shevardnadze's inner circle for misleading him about the situation in the country.
The president's victory was confirmed when the official election results were released on Thursday.
His group, the For A New Georgia bloc, won a 21% share, just ahead of the Revival Party and the opposition National Movement. Pre-election polls had shown that the bloc would lose most of its seats in parliament.
The United States State Department condemned the election, saying it was deeply disappointed with the results and with the Georgian leadership.
The BBC's Chloe Arnold in Tbilisi says the US has poured billions of dollars of aid into Georgia and invested in a key oil pipeline in the region.