Georgia's opposition coalition is leading in the popular vote after the country's parliamentary elections, according to early exit polls.
The Georgian Dream coalition, headed by billionaire Bidzina Ivanishvili, has claimed victory but so has the party of current President Mikheil Saakashvili.
However, the electoral system means that even if final results bear out the exit polls, the opposition may not have achieved a parliamentary majority.
Official results are due on Tuesday.
Thousands of cheering supporters of Georgian Dream gathered to celebrate in the capital Tbilisi.
"We have won! The Georgian people have won!" Mr Ivanishvili said in a speech broadcast on a Georgian TV station, the AFP news agency reports. Mr Ivanishvili said he expected his coalition to win 100 out of 150 parliamentary seats.
Early exit polls showed the opposition has a clear lead in votes for party lists, which determine who will win 77 out of 150 parliamentary seats.
In comments shown on Georgian TV, Mr Saakashvili conceded the opposition was ahead in that race but said his United National Movement was leading in the race for the 73 seats voted for on a first-past-the-post basis.
A spokeswoman for the United National Movement predicted that it would keep its majority.
Mr Saakashvili had sought to portray the election as a choice between his Western-leaning government, and a future in which he said Mr Ivanishvili would allow Russia to dominate the former Soviet republic.
Georgia's Central Electoral Commission (CEC) said in a statement that turnout had been around 61% and added voting had taken place "in a peaceful and transparent manner, with no serious incidents reported".
The CEC said it would announce results after 03:00 local time on Tuesday (23:00 GMT on Monday).
Earlier Mr Ivanishvili, who is Georgia's richest man, had staged a symbolic protest by refusing to vote, saying the authorities had "already resorted to very many violations".
The poll is being seen as the biggest test of President Saakashvili's popularity since he came to power in 2003.
Georgia's political system of different-sized constituencies means it is possible to win more seats in parliament, but with fewer votes in total, the BBC's Damien McGuinness in Tbilisi reports.
If the ruling party gets back into power that way, the opposition could well feel cheated of victory - and spark mass protests, our correspondent adds.
The government's reputation has taken a battering in recent weeks because of a prisoner-abuse scandal.
Videos broadcast on national television showed prison inmates being beaten and sexually abused by guards.
The scandal sparked street protests and allowed Mr Ivanishvili to portray the government as high-handed and uncaring.
The human rights group Amnesty International says many of Mr Ivanishvili's supporters were "fined, fired, harassed or detained for expressing their political views" during the election campaign.