None of the presidential candidates have garnered over 50% of the vote to score an outright victory, a Ukrainian-based election monitoring organization, Common European Cause, said.
It said Levan Gachechiladze, a unity opposition candidate, is ahead with 31% of the vote, Mikheil Saakashvili trailing him with 24.4%.
Polling stations closed at 8.00 p.m. local time (4.00 p.m. GMT) with a preliminary unofficial turnout of 46.4%.
Over 10,000 respondents at 200 polling stations across the country have been surveyed.
Presidential candidate Badri Patarkatsishvili, who is running third according to the exit poll, said he has enough evidence to prove that the elections have been rigged.
A source at Gachechiladze's election office also said that its observers have documented numerous violations, including the use of force against its monitors.
Georgia's Association of Young Lawyers, which conducts independent monitoring of the polls, said it has filed about 95 complaints with regional electoral commissions in the Georgian cities of Tbilisi, Kutaisi and Batumi.
Georgia's Central Electoral Commission (CEC) said it will carefully study each case of alleged ballot-rigging, a CEC spokesman, Irakly Porchkhidze, said.
"In the majority of cases, these reports have not been confirmed," Porchkhidze said.
Georgians also voted on the country's accession to NATO and were to decide on whether the parliamentary elections should be held in the spring and fall of 2008.
Saakashvili, who stepped down as president in November to launch his presidential campaign, faced six challengers - Levan Gachechiladze, a unity candidate of eight opposition parties; David Gamkrelidze, the New Right opposition party leader; Irina Sarishvili, of the Imedi political movement; Giorgy Maisashvili, the Party of the Future leader; Badri Patarkatsishvili, a foreign-based billionaire; and Shalva Natelashvili, leader of the Labor Party.
