Tens of thousands of Georgians on Saturday joined one of the biggest opposition rallies in years while the ruling party staged a rival mass meeting on the final campaign day before elections.

Huge crowds packed Tbilisi's central Freedom Square and main street to cheer billionaire opposition leader Bidzina Ivanishvili whose coalition is seeking to oust President Mikheil Saakashvili's ruling party in Monday's bitterly-contested vote.

His supporters marched on the square in large columns from several locations across the city, wearing his Georgian Dream coalition's T-shirts, carrying the bloc's banners and flags and chanting "Georgia! Georgia!"

"Saakashvili's system must be destroyed. The fate of the country is being decided at these elections," Ivanishvili told the rally.

"The day of a new government is coming," he said.

The rally ended with a raucous performance of the coalition's pop anthem by Ivanishvili's rapper son Bera, as hundreds of blue Georgian Dream balloons were released into the skies.

Thousands of Ivanishvili's supporters also staged a simultaneous rally in Georgia's second-largest city Kutaisi.

Meanwhile thousands of ruling party supporters gathered in the Black Sea resort city of Batumi, many waving Georgian flags, carrying campaign placards and chanting Saakashvili's nickname: "Misha! Misha!"

"My friends, we need your help so that nobody can stop the miracle we are all creating together in Georgia," Saakashvili told the cheering crowd.

Saakashvili also rallied around 60,000 people in Tbilisi on Friday.

Both sides are attempting to bolster support before the polls which are being held amid heightened tensions after Saakashvili's government was damaged by a prison torture scandal that sparked protests and international condemnation.

Ivanishvili charges that Saakashvili has established an authoritarian regime but the country's leader says the billionaire would stop modernising Georgia and throw it back into its corrupt and chaotic past.

The showdown has caused fears of post-poll unrest in the Western-backed state with a recent history of political turmoil and civil conflict.

Parliamentary delegations from the OSCE, Council of Europe, NATO and the European Parliament in Tbilisi to observe the polls issued a joint pre-election statement calling on all sides to "exercise restraint, renounce violence and not respond to provocation".

"Political leaders should be chosen through the ballot box and not on the streets," the statement said.

Both Saakashvili's United National Movement and the opposition Georgian Dream bloc have vowed to win the election after a campaign that was described by OSCE observers as "confrontational and rough".

Monday's polls are crucial for Georgia's future because its parliament and prime minister will take on more powers while the presidency's role be reduced under constitutional changes that come into force after Saakashvili's two-term rule ends in 2013.

The US and the EU, Georgia's main Western backers, have urged a free and fair vote in the impoverished Caucasus republic of 4.5 million people.