Iranians have taken to the polls in a general election that pits President Mahmoud Ahmadinejad against former prime minister and moderate, Mirhossein Mousavi. Four candidates are standing in the poll but incumbent Mr Ahmadinejad's strongest challenger appears to be the Mr Mousavi. The election outcome could help set the tone for Iran's relations with the West, which is concerned about Tehran's nuclear ambitions. A victory for Mr Mousavi could increase the prospects for Western investment in the country, according to analysts. Mr Ahmadinejad has been in office for four years, and his management of the Islamic Republic's oil exporting economy, which is suffering from high inflation and unemployment, has come under heavy scrutiny in recent months. Officials are expecting a high turnout from Iran's 46 million eligible voters. Preliminary results are expected on Saturday, and if no clear winner emerges, a run-off will be held on June 19 between the two front-runners. Supreme Leader Ayatollah Ali Khamenei, Iran's top authority, urged people to turn out for the election as he cast his ballot. He said: "Everybody go and everybody vote and act based on their judgement." Mr Ahmadinejad's opponents, who also include liberal cleric Mehdi Karoubi and former Revolutionary Guard leader Mohsen Rezaie, have urged the Interior Ministry and Khamenei to ensure there is no vote rigging.