Most of those inside the mosque were young boys and girls, local media said
At least 12 people have been killed and 160 wounded in an explosion at a mosque in southern Iran, local media reported.
Authorities in the city of Shiraz were investigating the blast, which some early reports had blamed on a bomb.
But a local police chief told Fars news agency that he had ruled out sabotage, and that "negligence" involving old war munitions might be the cause.
The explosion occurred at about 2100 (1630 GMT) on Saturday and was heard a mile (1.5km) away, the agency reported.
"The incident could have happened as a result of negligence. A while ago at this site there was an exhibition commemorating the [1980-1988] Iran-Iraq war," Commander Ali Moayeri, police chief of Fars province, told Fars news agency.
"The munitions left at the site could have the been the reason for this explosion," he added. The agency said he ruled out any act of sabotage.
Most of those inside the Hoseyniyeh Shohada mosque when the explosion took place were young boys and girls affiliated to the Rahpoyan-e Vesal Association, which "holds weekly meetings every Saturday regarding misguided groups, including Wahhabis and Bahais", Fars said.
The Irna news agency said the blast broke the windows of many nearby houses.
Television channels urged people in Shiraz to donate blood for the injured
Members of the Law Enforcement Force and the Basij militia placed a cordon around the mosque, while ambulance and fire crews worked to help the victims.
Television channels urged people in Shiraz to donate blood for the injured, adding that all nurses in the city had been called in on duty.
Shiraz, about 900km (560 miles) south of the capital Tehran, is a major tourist destination because of its closeness to a number of important ancient sites.
It has not been a target of the isolated bomb attacks which have occurred in Iran in recent years.
The last major bombing, in the south-eastern city of Zahedan in February last year, is believed to have been carried out by the Sunni Baluchi militant group, Jundallah.
Thirteen members of the Revolutionary Guard Corps were killed when a car bomb exploded next to the bus in which they were travelling.
The south-western city of Ahwaz, close to the Iraqi border, has seen sporadic anti-government violence since 2005, allegedly by its ethnic minority Arab population.