According to the television, a number of people were also injured in the blast, although details remain sketchy.

An Iranian opposition group, Mujadehin Khalq, issued a statement saying it was responsible for the blast in reprisal for the killing of a number of its fighters in clashes with government forces in western Iran late last year.

The Iranian news agency, quoting unidentified officials, said the blast occurred when explosives that were being taken into the court building for an investigation went off by accident.

However, other reports say the device was a briefcase bomb, which went off outside the office of the Revolutionary Prosecutor. The official is responsible for trying Iranians who are opposed to the revolution, including members of the outlawed Iraq-based group Mujahedeen Khalq.

Reformist President Khatami

In the 1980s, the Mujahideen were behind a series of huge explosions which killed a number of senior Iranian leaders. But a ruthless crackdown reduced their ability to operate. More recently their credibility inside Iran has been eroded by their close association with the Iraqis.

The BBC correspondent in Iran says that because the Mujahideen have not bombed the Iranian capital for some time, this might signal a change in tactics.

In recent months, Iran has been at the centre of an intensifying power struggle between the moderate reformists supporting President Mohammad Khatami and hard-line conservatives who oppose his social and political reforms.