TBILISI - Georgians on Sunday started voting in a presidential election to pick a successor to flamboyant moderniser Mikheil Saakashvili, with a trusted proxy of his billionaire foe Prime Minister Bidzina Ivanhishvili frontrunner to claim victory in the ex-Soviet state.
The poll brings the curtain down on US-ally Saakashvili's constitutionally allowed two terms in power and his acrimonious year-long political cohabitation with rival Ivanishvili, who has also pledged to step down shortly after the vote.
The election in the Western-backed Caucasus republic of some 4.5 million people will also usher in constitutional changes to shift a raft of key powers from the next president to the prime minister.
Polls in the run-up to the vote have put Giorgi Margvelashvili -- a little-known former education minister from Ivanishvili's Georgian Dream coalition - well ahead of ex-parliament speaker David Bakradze from Saakashvili's United National Movement (UNM).
Margvelashvili, 44, has said he is so certain of garnering the 50 per cent needed to win the first round -- which sees 23 candidates competing -- that he would pull out if the vote went to a second round.
But an aggressive challenge from former parliament chairwoman Nino Burjanadze -- along with a sizeable chunk of undecided voters -- could push the election to an uncertain second round.
The election campaign has been described by OSCE monitors as "notably calmer" than parliamentary polls last year that saw Ivanishvili, 57, wrest power from Saakashvili's party in Georgia's first smooth transition of power.
Georgia -- located on a strategic pipeline taking oil and gas from the Caspian Sea to Europe -- has gone through civil war, economic collapse and political turmoil since independence from the Soviet Union in 1991.
Ivanishvili's coalition will remain in control of the government whatever the result but the tycoon -- whose express aim was to dislodge Saakashvili -- has promised to stick to a pledge to name a successor as premier and step down shortly after the poll.
Ivanishvili insists Georgia's major priority will remain its goal of joining NATO and the European Union, a top Saakashvili ambition that enraged northern neighbour Russia.
But he also insists the government will press on with attempts to mend ties with Moscow which have been in tatters since the two sides fought a brief war in 2008 that saw Georgia effectively lose two breakaway regions, where Russia has now stationed thousands of troops.
While the election campaign may have been calm, Western allies have expressed concern over perceived selective justice that has seen a string of Saakashvili's close allies arrested since his party lost power.
Saakashvili, 45, has said he wants to remain active in politics but Ivanishvili -- who denies that there has been political witch-hunt -- has labelled him a "political corpse" and warned that he could face prosecution once his immunity ends when he leaves office.
During a tumultuous decade under Saakashvili -- who came to power after ousting Eduard Shevardnadze in the 2003 "Rose Revolution" -- Georgia cut corruption, built new infrastructure and revived the country's devastated economy.
Saakashvili's reforms though angered many who felt left out by the rush to change and took to the streets to protest his policies.
Ordering in riot police to brutally break up those rallies -- along with accusations Saakashvili allowed his cronies to grab key assets -- upset Western backers, who had hailed him as a model ex-Soviet democrat.
Over 3.5 million people are eligible to vote in the election which is monitored by international observers from the OSCE.
Polls close at 1600 GMT and preliminary results are expected to begin coming out overnight.