Iranian Supreme Leader Ayatollah Ali Khamenei accused US-backed Gulf Arab states of carrying out a shooting attack on a military parade that killed 25 people, almost half of them members of the country's elite Revolutionary Guards.
Khamenei ordered security forces to bring to justice those responsible for one of the worst assaults ever against the Revolutionary Guards, the most powerful military force in the country, which answers to him.
The allegation will likely ratchet up tensions with Iran's rival Saudi Arabia and its Gulf allies, which along with the United States have been working to isolate the Islamic Republic.
"This crime is a continuation of the plots of the regional states that are puppets of the United States, and their goal is to create insecurity in our dear country," Khamenei said in a statement published on his website.
He did not name the regional states he believed were to blame. Israel is also a key US ally opposed to Tehran.
An Iranian ethnic Arab opposition movement called the Ahvaz National Resistance, which seeks a separate state in oil-rich Khuzestan province, claimed responsibility for the attack.
Islamic State militants also claimed responsibility. Neither claim provided evidence. All four attackers were killed.
The assault, which wounded at least 70 people, targeted a viewing stand where Iranian officials had gathered in the city of Ahvaz to watch an annual event marking the start of the Islamic Republic's 1980-88 war with Iraq, state television said.
Iran has been relatively stable compared with neighbouring Arab countries that have grappled with upheaval since the 2011 uprisings across the Middle East.
Women and children died in the assault, state news IRNA agency reported.
The assailants had hidden weapons near the parade route several days in advance, said Brigadier General Abolfazl Shekarchi, a senior spokesman for Iran's armed forces.
"All four terrorists were quickly neutralised by security forces," Shekarchi told state television. "A four-year-old girl and a wheelchair-bound war veteran were among the dead."
Attacks on the military are rare in Iran.
Iran summoned the envoys of the Netherlands, Denmark and Britain on Saturday night over the shooting, accusing them of harbouring Iranian opposition groups in their countries. Some members of the ethnic separatist movements are based in the European countries.
Iranian foreign ministry spokesman Bahram Qasemi said Iran "asked their governments to condemn the attack and extradite people linked to it to Iran to be put on trial."
"It is not acceptable that these groups are not listed as terrorist organisations by the European Union as long as they have not carried out a terrorist attack in Europe," Qasemi was quoted as saying by IRNA news agency.