Television pictures showed shards of glass and piles of debris outside Shiraz's Mosque of the Martyrs as large crowds waited for news of relatives.

The death toll is expected to rise due to the severity of some people's injuries, officials said.

'No bomb'

"Initial surveys have proved that no bomb was involved and therefore there have been other probable causes," Ebrahim Azizi, the governor of Fars province, was quoted as saying by the state-run IRNA news agency.

Earlier, the Fars news agency, citing city officials had reported that "a powerful bomb" had exploded.

Al Jazeera's Alireza Ronaghi in Tehran said that the Iranian government may be being cautious about the cause of the blast with the country just two weeks away from the second round of elections.

"What we see here is that some of the officials have expressed concern about the nature of this explosion," he reported.

"We have heard the speaker of the parliament saying today that government officials and the authorities need to investigate deeply and properly what this incident was."

Jaber Baneshi, the prosecutor in Shiraz, told IRNA that "a judicial probe has been launched to determine the cause of the explosion and the possibility of sabotage".

'Package planted'

However, Mohammad Anjavinejad, the preacher who was addressing the crowd at the mosque, reportedly cast doubt on the accident theory, saying that the force of the blast and the presence of an individual who planted a package in the building suggested otherwise.
"Some parties are trying to show this was an accident to portray the city as safe. But it is their duty to implement security"

Mohammad Anjavinejad, preacher
"Some parties are trying to show this was an accident to portray the city as safe. But it is their duty to implement security," he told the Alef news website.

The explosion took place at around 9pm local time (1630GMT) on Saturday during evening prayers.

State television urged people in Shiraz to donate blood for the wounded and said that all nurses in the city had been called to report for work.

Fars said the force of the blast shook houses more than one kilometre away.

"There was a huge blast and the whole place lit up. Everyone started shouting and screaming and tried to help each other," Marzian Mohammadnejad, a witness, told Iran's English language Press-TV.

Bahai faith banned

Fars reported that the mosque hosted weekly speeches denouncing Wahhabism - a version of Sunni Islam - and the outlawed Bahai faith. Iran is a predominantly Shia country.

Iran had been the cradle of the Bahai faith in the middle of the 19th century. After the Islamic revolution in 1979, the faith was banned and the country's constitution does not recognise it as a religious minority.
Last year, Bahai communities abroad said some of followers of the faith were detained in Shiraz while working with poor communities there.
Shiraz is close to a number of ancient sites popular with tourists.
Iran suffered its last major bombing in February 2007 in the southeastern city of Zahedan.
Thirteen members of Iran's elite Revolutionary Guard died in that blast, and a Sunni group called Jundallah, or God's Brigade, claimed responsibility for the attack.