The state-run news agency in Iran has confirmed that the head of the only opposition group that's tolerated in Iran, Ibrahim Yazdi, has been arrested. The agency IRNA gave no reason for the arrest, and said no further information was available on what legal steps would be taken against Mr Yazdi, who once served as the Islamic Republic's foreign minister. On Monday, Mr Yazdi's group, the Freedom Movement, said he'd been taken in for questionning and then transferred to prison. Our regional reporter, Caroline Hawley, looks at what might be behind the arrest.

Mr Yazdi is one of the most prominent critics of the conservative clergymenwho've ruled Iran for the past eighteen years. In the past, he's been tolerated, perhaps because of the key role he played in the Islamic Revolution of 1979.

But President Khatami's overwhelming election victory against his conservative opponents in May, has put the conservatives on the defensive. And they're now fighting back.

In Iran's complex political system, the conservatives, led by the Supreme Leader, Ayatollah Khamenei, remain enormously powerful and the President is subordinate to the Supreme Leader, who controls Iran's security forces and judiciary. But since May growing numbers of people have begun, for the first time, to openly challenge Iran's political set-up.

President Khatami's supporters have called for some of the vast powers enjoyed by the Supreme Leader to be transferred to the President. But Ayatollah Khamenei's supporters are now hitting back.

Last month, when a dissident cleric, Ayatollah Montazeri, questionned the authority of Ayatollah Khamenei, his home in the holy city of Qom was attacked by thousands of angry demonstrators. On Saturday, Ibrahim Yazdi signed a letter demanding that the rights of the dissident cleric be respected.