The number of casualties is uncertain but several people are said to have been injured.

One report says the device was a briefcase bomb which, according to witnesses, exploded close to the revolutionary prosecutor's office in the east of the city.

The prosecutor's office tries Iranians who are opposed to the revolution, including members of the outlawed Iraq-based group Mujahedeen Khalq.

Reformist President Khatami

However, no-one has claimed responsibility for the attack.

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.