The bomb detonated during evening prayers in the mosque
A bomb in a mosque in south-east Iran has killed at least 19 people and injured 60, the governor of Sistan-Baluchestan province said.
The explosion happened in Zahedan, the provincial capital, at the time of evening prayer, Ali Mohammad Azad told Iranian state TV.
He said "terrorists", who had planned to detonate more bombs, were arrested.
The province is often the scene of lawlessness, including clashes between police and drug dealers or militants.
Zahedan is a mainly Sunni Muslim city in a mostly Shia country.
Part of the Amir al-Mohini mosque was destroyed, the official news agency Irna reported.
Rescue teams were taking out the bodies of the dead and injured.
"It was a terrorist attack and the bomb was exploded by a terrorist," Mr Azad said, according to Irna, adding that members of a terrorist group had been arrested.
Mr Azad said "bandits and terrorists intended to disturb the order in the province before the election considering the insecurity in the eastern neighbouring countries".
Although it occurred in a remote region, the explosion comes at a time of heightened political sensitivity nationally, with just over two weeks before the
first round of the presidential election
Fars news agency quoted witnesses saying the incident had been a suicide attack, and that a second bomb had been defused near the mosque. The reports could not be verified.
Thursday was a public holiday marking the death of the Prophet Muhammad's daughter, Fatima.
Sistan-Baluchestan is one of the most deprived regions in Iran.
Its location on the borders of both Afghanistan and Pakistan make it a key route in the drugs trade.
Despite Iran's best efforts, a huge proportion of the world's opiates, such as heroin and morphine, are smuggled by heavily armed drugs gangs, often in large convoys.
There are also a number of militants in the area, many of them with links to the drugs gangs, and clashes with the security forces are common.
Two years ago at least 11 people, including members of Iran's Revolutionary Guard, were
killed by a bomb in Zahedan.
Foreigners are strongly advised to avoid the area, and a recent visit by a number of diplomats was accompanied by extremely heavy security, says the BBC's John Leyne in Tehran.
The insurgency is linked with the area's large Sunni population - at odds with Iran's Shia-led government.
But the Iranian government also accuses the US and Britain of supporting the militants.