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".
Terrorists recruited, trained, armed & paid by a foreign regime have attacked Ahvaz. Children and journos among casualties. Iran holds regional terror sponsors and their US masters accountable for such attacks. Iran will respond swiftly and decisively in defense of Iranian lives. pic.twitter.com/WG1J1wgVD9— Javad Zarif (@JZarif) September 22, 2018
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.
Fars news agency said the attack started at 09:00 local time (06:30 BST), lasted about 10 minutes, and appeared to involve four gunmen.
The attackers fired at civilians and attempted to attack military officials on the podium, Fars reports.
Civilians including women and children, who were watching the military parade, were among those killed, Irna news agency said.
The victims included a four-year-old girl and a military veteran in a wheelchair, a military spokesman said.
Local journalist Behrad Ghasemi told AFP that firing continued for between 10 and 15 minutes and said at least one of the attackers was wearing a Revolutionary Guards uniform.
"First we thought it's part of the parade, but after about 10 seconds we realised it was a terrorist attack as bodyguards [of officials] started shooting," he said.
"Everything went haywire and soldiers started running. I saw a four-year old child get shot, and also a lady," he added.
All four attackers were killed, state media said.
Iran is marking the anniversary of the beginning of the 1980-88 war with Iraq with several military parades across the nation.
How will the US react?
By Siavash Mehdi-Ardalan, BBC Persian
There have been two conflicting claims of responsibility: one from a low profile Arab militant group in Iran's Khuzestan region and one from IS. It makes some difference.
The former would suggest a resurgence of separatist militancy after a seven-year lull. If it was IS, it would represent a failure by Iran's intelligence community to prevent a second major IS attack in its soil.
Iran has not provided any evidence of foreign collusion but has vowed revenge. The Saudi reaction and more importantly the wording of the US administration's response may prove important as leaders of all three countries are set for a possible diplomatic clash at the UN General Assembly next week.
Who is behind the attack?
There have been conflicting claims.
A spokesman for the Ahvaz National Resistance, an umbrella group that claims to defend the rights of the Arab minority in Khuzestan, said the group was behind the attack.
The spokesman did not say whether the group had links to other countries.
IS's Amaq agency has also claimed it carried out the attack. However the group provided no evidence that it had been involved.
IS has carried out a major attack in Iran before. In June last year, suicide bombers attacked parliament and the mausoleum of the Islamic Republic's founder Ayatollah Khomeini, killing 18 people.
Iranian government and military officials have pointed the finger at Gulf states, the US and Israel, with all of whom Iran has longstanding tensions.
A Revolutionary Guards spokesman claimed the attackers were "trained and organised by two Gulf countries" and had ties to the US and Israel.
The US and Saudi Arabia accuse Iran of supporting Houthi rebels in the conflict in Yemen, where Saudi Arabia and the United Arab Emirates are fighting on the side of the internationally-recognised government of President Abdrabbuh Mansour Hadi.