Earlier, van-loads of riot police had been seen arriving at Tbilisi's parliament square.
Opposition leaders have appealed to the government not to use violence to break up mass protests.
Police used rubber bullets and tear gas to break up the last mass protests in Tbilisi in November 2007.
Claims of plot
"I don't think that it should be a surprise that after we lost 20% of Georgian territory and have no democracy in the country, we are asking for the resignation of the president," said Nino Burjanadze.
Ms Burjanadze was formerly an ally of Mr Saakashvili but now leads the opposition Democratic Movement-United Georgia party.
The party said 60 members, who had been planning to attend the demonstrations, were arrested overnight.
An interior ministry spokesman said that was "not true".
Our correspondent says both opposition and government figures have accused one another of planning to use violence in Thursday's rallies.
Video footage was recently released by the government allegedly showing a group of opposition supporters planning a disturbance at the protests.
The government accused the men of trying to provoke the government into using force.
Undercover video filmed by Georgian police
Interior Minister Vano Merabishvili said the government would "not intervene or impede members of the protest in expressing their will freely" but indicated that the authorities could take action if they deemed it necessary.
"My position does not give me the liberty to exclude anything, but my mood tells me there will not be violence," he told Reuters.
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