An explosion at a mosque in the city of Zahedan in southeast Iran has killed 30 people and wounded 60.
The semi-official news agency ILNA reported the blast was a suicide bombing.
The FARS news agency reports security forces later discovered and defused a second bomb near the mosque.
The attack was carried out on a public holiday honouring the first Shi'ite Imam, Ali Ebne-Abitaleb, after whom the mosque is named. Zahedan is a mostly Sunni city.
The provincial governor told state television the explosion occurred at about 7:45pm (1515 GMT) on Thursday when many people were inside the mosque for prayers.
Zahedan is the capital of Sistan-Baluchestan province which shares a border with Pakistan. There are frequent clashes between police and drug dealers and bandits.
The presidents of Iran, Pakistan and Afghanistan met in Tehran for their first summit on Sunday, in an effort to improve cooperation in fighting terrorism and drug trafficking and tackling other regional security problems.
Iran is also preparing for a presidential election on 12 June. President Mahmoud Ahmadinejad is seeking a second term and faces three challengers.