A blast at the Ali Ibn Abi Taleb mosque in the southern province of Sistan-Baluchestan along the border with Pakistan killed as many as 21 people and wounded at least 70 others, the Islamic Republic of Iran Broadcasting agency reports.
The mosque honors the cousin and son-in-law of the Prophet Mohammad. The attack came as mourners were holding prayers during a national day of mourning commemorating the death of Fatimah Zahra, the daughter of Mohammad.
The report says Iranian intelligence officials arrested several suspects in the city of Zahedan, the provincial capital.
Ali Mohammad Azad, the provincial governor, said officials feared additional attacks in the region given the proximity to violence in Afghanistan and Pakistan.
Security forces claim to have defused a second bomb while examining the mosque in the wake of the attack. Authorities believe a single person carried out the attack, though reports in the Iranian media refer to the incident as a terrorist attack.
Ayatollah Ahmad Khatami said the attacks bore the tell-tale signs of U.S. and Israeli involvement, the Fars news agency reports.
Iranian officials called for an additional three days of mourning to mark the attack at the mosque.