A Shia cleric known for his stance against Wahhabism, a strict version of Sunni Islam, has been killed in the south-western Iranian city of Ahwaz.
Reports said Hojjat ol-Eslam Hesham Seymari was shot dead in his home at night by two men claiming to know him.
Wahhabism is practised throughout the Arabian Peninsula, most notably by the ruling family of Saudi Arabia.
Ahwaz, home to a large community of ethnic minority Arabs, has seen much anti-government tension since 2005.
Iran has publicly hanged 10 men convicted of taking part in a series of deadly bombings in the city last year, which were blamed on Arab separatists allegedly backed by the UK.
The separatists have accused Iran's Persian majority of discrimination.
Reporting Hojjat ol-Eslam Seymari's death on Tuesday, the hardline Iranian newspaper Keyhan described the cleric as a "skilful speaker who brought awareness to the youth about deviating movements, especially Wahhabism".
Hojjat ol-Eslam Seymari supervised 20 mosques and "stood against the Wahhabis' investment and strong onslaught in the city of Ahvaz", the paper added.
The representative of Iran's Supreme Leader in Khuzestan Province said the cleric had been martyred "at the hands of the mercenaries of Satan".
"Our enemies should know that his martyrdom will guide the nation, keep alive the values and principles of the exalted Muslim faith," Mohammad Ali Jazayeri said in a statement.
"Islam will remain alive and the enemies of Islam will die and receive a fitting punishment."
Wahhabism was founded by Mohammed Bin Abdul-Wahhab in 18th Century Arabia as an Islamic reform movement which felt that the local practice of Islam had lost its original purity.
The movement formed an alliance with the founders of the Saudi dynasty which continues today. A modern militant version of Wahhabism is espoused by Osama Bin Laden's al-Qaeda organisation.
Followers do not describe themselves as Wahhabis, preferring to call themselves Muwahhidun (unitarians). Some view Shia Islam as a heterodox sect.