The bomb explosion during a military parade in the city of Mahabad on Wednesday also wounded 80 people.
Iranian authorities blamed the blast on "anti-revolutionary" militants backed by foreign enemies, including the United States and Israel.
"The group that carried out this terrorist attack has been arrested ... and hopefully they will be punished ..." Moslehi was quoted as saying.
No group has claimed responsibility for the blast, which occurred during the "Sacred Defense" celebrations, an annual ceremony for the armed forced to commemorate Iran's eight-year war with Iraq in the 1980s.
Moslehi gave no details of those arrested, saying "the blast targeted unity among Shiites and Sunnis in the country."
The government announced three days of mourning in Mahabad, the capital of a short-lived Soviet-backed "Republic of Kurdistan" in 1946, which was crushed within a few months.
It was also the center of a Kurdish uprising shortly after the 1979 Iranian revolution.
Iranian media have often reported clashes between the elite Revolutionary Guards and Kurdish guerrillas said to be members of the Party of Free Life of Kurdistan, an offshoot of the Kurdistan Workers Party which began an armed campaign in 1984 for an ethnic homeland in southeast Turkey and northwest Iran.
Several armed groups hostile to the establishment are active in Iran, including Kurdish separatists in the northwest, Baluch militants in the southeast and some Arabs in the southwest.
The militant group Jundollah, which Iran says is linked to Al-Qaeda, is the most active. It claimed responsibility for a double suicide attack that killed 28 people, including Revolutionary Guards, on July 15 in retaliation for the execution of its leader.
