Several foreign nationals were detained with some 100 kilograms of explosives.
Iran says it has broken up an ISIS plot to bomb commemorations marking the Shiite religious mourning period of Ashoura.
Iran's official IRNA news agency quoted Intelligence Minister Mahmoud Alavi late Wednesday as saying several foreign nationals were detained with some 100 kilograms (220 pounds) of explosives. State television also reported the arrests.
Alavi said the suspects planned to attack events in Iran's southern Fars province. He referred to the suspects as "takfiri terrorists," a term used by Iranian officials to describe militant Sunni fundamentalists like the ISIS group.
There was no other information immediately released. Iran has helped Shiite militias fighting against ISIS in Iraq.
In June, Iran said it broke up one of the "biggest terrorist plots" ever on its soil by Sunni extremists.