Iran has arrested the group behind a blast that killed 12 people in the northwestern region, state television quoted Intelligence Minister Heidar Moslehi as saying on Thursday.
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 of the establishment, 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 Defence" 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 Sh'ites and Sunnis in the country".
The government announced three days of mourning in Mahabad.