Iranian leaders have accused US-backed Gulf states of being behind an attack on a military parade that killed 25 people, including a child.
Supreme Leader Ayatollah Ali Khamenei said "puppets of the US" were trying to "create insecurity" in Iran.
Gunmen opened fire at Revolutionary Guard troops and officials in the south-western city of Ahvaz.
Earlier an anti-government Arab group, Ahvaz National Resistance, and Islamic State (IS) both claimed the attack.
However neither group provided evidence to show they were involved.
Earlier Foreign Minister Javad Zarif blamed "terrorists paid by a foreign regime", adding that "Iran holds regional terror sponsors and their US masters accountable".
Iran has summoned diplomats from the UK, the Netherlands, and Denmark, accusing their countries of harbouring Iranian opposition groups, state news agency Irna reports.
"It is not acceptable that these groups are not listed as terrorist organizations by the European Union as long as they have not carried out a terrorist attack in Europe," said foreign ministry spokesman Bahram Qasemi.
Reports say nearly half of those killed were members of the Revolutionary Guard, who are under Mr Khamenei's command.
Mr Khamenei did not name the "regional states" that he believed were behind the attack.
However Iran has previously accused its regional rival, Saudi Arabia, of supporting separatist activity amongst Iran's Arab minority.