The rally is the latest protest against Saakashvili, a stalwart U.S. ally, who Opposition leaders said their main demand at Friday's rally would be the reversal of a decision to move back next year's parliamentary election by several months.
Political turmoil in the former Soviet nation was sparked by sensational allegations against Saakashvili by his former defense minister, Irakli Okruashvili, who was effectively sacked late last year.
Okruashvili, a hawkish, one-time Saakashvili supporter, was arrested but then freed on multimillion-dollar (-euro) bail last month after he retracted allegations accusing Saakashvili of corruption and a murder plot. The accusations and arrest set off large protest rallies in the Georgian capital.
The Interior Ministry said Thursday that Okruashvili flew to Germany on Wednesday for unspecified medical treatment. Opposition leaders claimed that he was forcibly put on the flight so he could not lead the opposition protests.
Opposition activists claimed that Georgian police had set up roadblocks in several parts of the country to keep anti-government protesters from traveling to Tbilisi for the demonstration.
The Georgian parliament last year approved Saakashvili's proposal to hold the next parliamentary vote in the fall of 2008 instead of the spring.
Saakashvili also proposed moving the presidential vote, scheduled for January 2009, and holding the two elections simultaneously as a way of saving money.
The opposition has called the proposal a ploy by Saakashvili to hold the presidential election early, before his falling popularity has an effect at the polls.