Updated

Iran was holding funerals Monday for the victims of the weekend terror attack on a military parade in the southwestern city of Ahvaz, the deadliest attack in the country in nearly a decade.

Thousands of mourners gathered at the city's Sarallah Mosque on the Taleghani junction, carrying caskets in the sweltering heat.

Others, mainly young people wearing ethnic clothes of the region's Arab minority, held large photographs of those slain at Saturday's parade in Ahvaz, the Khuzestan provincial capital, where militants disguised as soldiers had opened fire at marching troops and onlookers. Of the 25 people killed, 12 were from Ahvaz and the rest from elsewhere in Khuzestan.

The procession walked down the Naderi and Zand Streets, many weeping and beating their chests, a traditional way of showing grief. Mourners played drums, cymbals and horns, according to local Arabic custom.

Cries and wails erupted when the casket of a local Ahvaz hero, 54-year-old Hossein Monjazi, a disabled war veteran and Revolutionary Guard member who had lost a leg and a hand in the Iraq-Iran war of the 1980s, was brought out.

Monjazi was in the wheelchair watching the parade when the gunshots erupted and was unable to find shelter from the hail of bullets.

Speaking at the funeral ceremony, Revolutionary Guard's acting commander Gen. Hossein Salami vowed revenge against the attack's perpetrators and what he called the "triangle" of Saudi Arabia, Israel and the United States

Arab separatists have claimed the assault, which killed 25 and wounded 60, including Guard members and soldiers. Iranian officials have blamed the separatists for the attack. The Islamic State group also claimed responsibility for the attack, but offered no clear evidence it carried out the assault.

President Hassan Rouhani on Sunday accused an unnamed U.S.-allied regional country of supporting the perpetrators. Iran's Foreign Ministry summoned Western diplomats, accusing them of allegedly providing havens for the Arab separatists behind the attacks.

The Ahvaz attack has further shaken Iran, already facing turmoil in the wake of the American withdraw from Tehran's nuclear deal with world powers.

Rouhani's remarks could refer to Saudi Arabia, the United Arab Emirates or Bahrain — close U.S. military allies that view Iran as a regional menace over its support for militant groups across the Middle East.

"All of those small mercenary countries that we see in this region are backed by America. It is Americans who instigate them and provide them with necessary means to commit these crimes," Rouhani said before leaving for the U.N. General Assembly in New York.