What is at stake?
The stakes are huge for Iranian society first, but also for world powers, which have been fretting over Tehran’s increased radicalism and a nuclear programme that they suspect is ultimately aimed at producing nuclear weapons, even if Iran denies this. So much of the world would be relieved if the election deprived Mahmoud Ahmadi-Nejad, the fundamentalist president, of a second term. Although Ayatollah Ali Khamenei, supreme leader, has ultimate decision-making power, Mr Ahmadi-Nejad has shown a president’s influence can be significant.

 How has Moussavi emerged as the main challenger?
With determined help from a large part of the political establishment which is desperately seeking to deprive Mr Ahmadi-Nejad of another four years. Mir-Hossein Moussavi, prime minister in the 1980s, has won the support of various disenchanted groups. And there are many, including the educated middle class, which sees the incumbent as an embarrassment, and fears that his erratic, populist economic policies have wrecked the economy.