The figure given by provincial governor Ali Mohammad Azad to the official IRNA news agency was lower than a toll of 30 cited by another Iranian news agency hours after the blast in Zahedan city, near the border with Pakistan, on Thursday evening.
The authorities say they have arrested members of a "terrorist group" behind the blast. The predominantly Shi'ite Muslim country holds a presidential election on June 12.
About 80 people were admitted to hospital with injuries, Azad said. The semi-official ILNA news agency said on Thursday the blast was a suicide bombing.
The explosion occurred in a prominent mosque in Zahedan, the main city of Sistan-Baluchestan province, a turbulent border region where a Sunni rebel group has been operating. Iran says the group is part of the Sunni Islamist al Qaeda network.
In April, the intelligence minister said Iran had arrested a group of people linked to Israel who were planning bombings ahead of the June election, in which hard-liner President Mahmoud Ahmadinejad is seeking a second four-year term.
Iran often accuses Israel and the United States, its two arch foes, of seeking to undermine the Islamic Republic, especially in sensitive border areas.
"Again blood of the oppressed is on the hands of terrorist criminals and those who are fed by the global arrogance," a provincial representative of Supreme Leader Ayatollah Ali Khamenei said in a statement quoted by ISNA news agency.
Iranian leaders often refer to the United States as the "Great Satan" and "Global Arrogance." ISNA did not name the official who made the statement.
(Reporting by Zahra Hosseinian; writing by Fredrik Dahl; Editing by Robert Woodward)
