The death toll in twin suicide bombings near a mosque in the country's southeast has risen to 38, it has been reported.
The IRNA news agency said the dead included women and children.
Wednesday's attack took place in the port city of Chahbahar, near the border with Pakistan.
The bombs targeted a group of worshippers at a mourning ceremony a day before Ashoura, which commemorates the seventh century death of the Prophet Muhammad's grandson Hussein, one of Shiite Islam's most beloved saints.
The area of Iran is home to an armed Sunni militant group, Jundallah, or Soldiers of God, which has waged sporadic attacks to fight alleged discrimination against the area's Sunni minority in overwhelmingly Shiite Iran.
There was no immediate claim of responsibility, but the use of multiple suicide attackers to target Shiite worshippers is a tactic the group has employed in the past.
One of the attackers detonated a bomb outside the mosque and the other struck from inside a crowd of worshippers, state TV reported.
Mahmoud Mozaffar, a senior Iranian Red Crescent Society official, said emergency services had been put on alert over the past few days because of anonymous threats, according to another news agency, ISNA.
In July, two suicide bombers blew themselves up at a mosque in the same province, Sistan-Baluchestan, killing at least 28 people. Jundallah said that attack was revenge for the execution of its leader, Abdulmalik Rigi, in June.
That strike in the provincial capital, Zahedan, also targeted Shiite worshippers during a holiday, the birthday of Hussein, the prophet's grandson.