Georgia voted on Monday in parliamentary polls that pit President Mikheil Saakashvili\'s party against a billionaire tycoon who poses the first big challenge to his rule and has warned of mass protests if the elections are rigged. The showdown between Saakashvili\'s party and an opposition coalition led by Bidzina Ivanishvili has turned increasingly bitter after a scandal over torture in prisons sparked protests across the ex-Soviet state. Ivanishvili has threatened mass protests if Western observers don\'t declare a fair vote and his Georgian Dream bloc mobilised tens of thousands of supporters at the weekend for one of the biggest rallies Tbilisi has ever seen. \"It is a day of historic importance for Georgia. The fate of the Georgian state is being decided,\" Saakashvili, whose campaign was hurt by the prison abuse scandal, said after casting his ballot in Tbilisi.Ivanishvili said the elections would give Georgia a new government despite his concerns that they were being rigged in favour of the ruling party. \"For the first time Georgians are making a \'nearly\' democratic choice. I say \'nearly\' because the authorities have already committed lots of violations,\" he said outside a polling station in Tbilisi. The highly-polarised campaign in the country of 4.5 million people, described by OSCE election monitors as \"confrontational and rough\", has raised fears of post-poll unrest. Georgia\'s main backers, the US and the EU, have called for a fair vote and emphasised that democratic progress is crucial for the small Caucasus republic\'s ambitions to join Western institutions like NATO. Saakashvili\'s party has dominated Georgia since the charismatic lawyer rose to power after the 2003 \"Rose Revolution\" that ousted the country\'s former leader, the wily ex-Soviet foreign minister Eduard Shevardnadze. The president\'s United National Movement occupies 119 of the 150 seats in the outgoing parliament but Saakashvili has promised the \"most free, most transparent\" polls since Georgia became independent in 1991. The elections 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 Saakashvili\'s two-term rule ends in 2013. Turnout was 25.5 percent at 0800 GMT, four hours after polls opened, the Central Election Commission said. \"These elections are of colossal importance. The unjust rule of (Saakashvili\'s) United National Movement must end,\" one voter, engineer Nodar Khinsalishvili, told AFP 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. Ivanishvili has accused Saakashvili of establishing an authoritarian regime but the Georgian president says the billionaire would stop the government\'s modernisation programme and throw the country back to its corrupt past. 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 Saakashvili and 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.