The president's ruling party occupies 119 of the 150 seats in the outgoing parliament but Mr Saakashvili has promised the "most free, most transparent" polls since Georgia became independent in 1991.
Mr Ivanishvili however has alleged that the electoral environment was rigged in favour of the ruling party and has threatened protests if Western observers don't declare a fair vote.
The polls are crucial for Georgia's future because its parliament and prime minister will become stronger and the presidency's powers will be significantly reduced under constitutional changes that come into force after Mr Saakashvili's two-term rule ends in 2013.
"These elections are of colossal importance. The unjust rule of [Saakashvili's] United National Movement must end," engineer Nodar Khinsalishvili, said at a polling station in Tbilisi.
"Two giants are competing in these elections. I support the United National Movement - Saakashvili's reforms must continue, it is vital for Georgia," said another voter, musician Maya Chigladze.
Both Mr Saakashvili's United National Movement and Mr Ivanishvili's Georgian Dream bloc have staged shows of strength in recent days with mass rallies that brought tens of thousands onto the streets.
Mr Ivanishvili has accused him of establishing an authoritarian regime.
"Saakashvili's system must be destroyed," the tycoon told a huge rally Saturday.
However the Georgian president says the billionaire would stop the government's modernisation programme and throw the country back to its corrupt past.
Poverty and unemployment were key campaign issues, with the rivals offering populist promises of big spending on social welfare and job creation.
Georgia is a mountainous republic located on a strategic pipeline route that takes Caspian Sea oil and gas to Europe.
Since independence from the Soviet Union, it has gone through economic collapse, civil war and repeated outbreaks of political unrest which have seen two presidents deposed.
Its most recent conflict, a brief war with arch-foe Russia in 2008, saw it effectively lose two Kremlin-sponsored breakaway provinces where Moscow has now stationed thousands of troops, casting a shadow over Georgia's security.
Both Mr Saakashvili and Mr Ivanishvili say they want EU and NATO membership for Georgia, although the billionaire also wants to mend relations with Russia.
A total of 14 parties and two blocs are standing for the 150-seat parliament which is elected for a four-year term. The electoral system is a combination of single-mandate constituencies and seats determined by proportional representation.
More than 3.6 million people are eligible to vote in the elections which are being monitored by international observers from the OSCE. Polls close at 1600 GMT.
Source: AFP
Mr Ivanishvili however has alleged that the electoral environment was rigged in favour of the ruling party and has threatened protests if Western observers don't declare a fair vote.
The polls are crucial for Georgia's future because its parliament and prime minister will become stronger and the presidency's powers will be significantly reduced under constitutional changes that come into force after Mr Saakashvili's two-term rule ends in 2013.
"These elections are of colossal importance. The unjust rule of [Saakashvili's] United National Movement must end," engineer Nodar Khinsalishvili, said at a polling station in Tbilisi.
"Two giants are competing in these elections. I support the United National Movement - Saakashvili's reforms must continue, it is vital for Georgia," said another voter, musician Maya Chigladze.
Both Mr Saakashvili's United National Movement and Mr Ivanishvili's Georgian Dream bloc have staged shows of strength in recent days with mass rallies that brought tens of thousands onto the streets.
Mr Ivanishvili has accused him of establishing an authoritarian regime.
"Saakashvili's system must be destroyed," the tycoon told a huge rally Saturday.
However the Georgian president says the billionaire would stop the government's modernisation programme and throw the country back to its corrupt past.
Poverty and unemployment were key campaign issues, with the rivals offering populist promises of big spending on social welfare and job creation.
Georgia is a mountainous republic located on a strategic pipeline route that takes Caspian Sea oil and gas to Europe.
Since independence from the Soviet Union, it has gone through economic collapse, civil war and repeated outbreaks of political unrest which have seen two presidents deposed.
Its most recent conflict, a brief war with arch-foe Russia in 2008, saw it effectively lose two Kremlin-sponsored breakaway provinces where Moscow has now stationed thousands of troops, casting a shadow over Georgia's security.
Both Mr Saakashvili and Mr Ivanishvili say they want EU and NATO membership for Georgia, although the billionaire also wants to mend relations with Russia.
A total of 14 parties and two blocs are standing for the 150-seat parliament which is elected for a four-year term. The electoral system is a combination of single-mandate constituencies and seats determined by proportional representation.
More than 3.6 million people are eligible to vote in the elections which are being monitored by international observers from the OSCE. Polls close at 1600 GMT.
Source: AFP