"The official media is widely distrusted ... so irrespective of who really carried out that bombing, I am sure that the protesters on the street will feel that the government has simply engineered something to blame them for something else," he said.

'Forced confrontation'

But he said that it was possible that the People's Mujahidin of Iran or Mujahedeen-e-Khalq (MEK), a group which calls for the overthrow of the Islamic republic, could have carried out the attack.

In depth



 Video: Iran supreme leader in 'power struggle'
 Video: Iran's 'citizen journalists'
 Video: Iran steps up net censorship
 Video: Iranians go online to evade curbs
 Video: The struggle for power
 Video: Rival protests continue in Iran
 Video: Iranians rally in Europe

 Iran's Ayatollah under threat?
 Mousavi sees election hopes dashed
 Iran writer on poll result
 Mousavi's letter to the people
 Iran poll result 'harms US hopes'
 West concerned by Iran fraud claims
 What next for Iran?
 The Iranian political system
 Riz Khan: Iran's disputed election
 Inside Story: Iran election recount
 Inside Story: Iran's political future

 Your media: submit your clips of the protests to Al Jazeera 

"They have dwindled in number and they have certainly dwindled as far as credibility is concerned, but you can't write off the fact they may still have some capability," he said.

MEK took part in the Islamic revolution in 1979 that brought Khomeini to power, but their ideology, a blend of Marxism and Islamism, put it at odds with the revolutionary government.

In 1981, the group was driven from its bases on the Iran-Iraq border and resettled in Paris.

It has previously targeted Iranian government officials and government facilities in Iran and abroad.

Mehrdad Khonsari, a former Iranian diplomat, told Al Jazeera: "We have to wait and see who instigated that blast ... but the whole point is the regime has forced a situation whereby there is a confrontation.
 
"They have tried to intimidate the protesters from not coming and not embarking on the course which they have. That has apparently failed. So the question is, how much more can this situation escalate?
 
"If this blast has led to the death of a number of people, this will infuriate the masses and it will propel them to want to engage. This is how revolutions happen."

The explosion took place as police fired tear gas and water cannon at thousands of protesters gathered in Tehran to protest over the disputed re-election of Mahmoud Ahmadinejad, the Iranian president.

Foreign television crews are operating under severe restrictions in Iran and many non-Iranian journalists have been prohibited from working in the country.