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is set to hold a memorial service on Wednesday for a Shia Muslim a day earlier by a suspected Sunni extremist in the revered Imam Reza shrine in the northeastern city of Mashhad.

The chief suspect in the bloody attack was named Abdolatif Moradi, 21, an ethnic Uzbek who had entered Iran illegally via the border a year ago, the Tasnim news agency reported.

Moradi had "worked in transport" in a poor city district and had been active on social media using pseudonyms including Abdolatif al-Salafi to "spread takfiri ideology and confront Shias".

Iran's blamed the knife attack, which wounded two other clerics, on the influence of US-based "takfiri" -Ìýa term used for Muslims who brand others as apostates, condemning them to death, and usually referring to Sunni extremists.

Raisi -Ìýwho once chaired the charitable foundation that runs the gold-domed Imam Reza shrine - instructed Iran's intelligence ministry to probe the killing of the cleric Mohammad Aslani, who held the rank of hojatoleslam.

The assailant struck on Iran's third day of the holy month of as large crowds of worshippers had gathered in the courtyard of the shrine of Imam Reza, one of the most revered figures in Shia Islam.

Authorities arrested six suspected accomplices, including the chief suspect's two brothers, after the attack in Mashhad, Iran's second largest city with more than three million people.

The assailant stabbed one of the victims "20 times", the Tasnim report said.

Flowers were placed at the scene of the attack, in the courtyard of the mausoleum of Imam Reza, the eighth of the 12 holy imams in Shia Islam, Iran's state religion since the 16th century.

The Fars news agency released a picture of the three victims seated together in Mashhad, naming the two wounded clerics only as Pakdaman and Daraei.

The memorial service for Aslani was to take place in the evening, state news agency IRNA reported. His body will be carried by shrine officials, who will circle the courtyard several times while worshippers read the .

He will be buried on Thursday in a square reserved for martyrs in the shrine's courtyard.

The head of media at the mausoleum, who gave his name as Mr Rostamzadeh, told AFP that "after the knife attack, the assailant was arrested by security services and 15 minutes later everything was back to normal.

"People were absolutely not afraid - on the contrary, it aroused religious fervour," he said. "There was no closure or interruption."

"There are a lot of people, the shrine is full of worshippers and pilgrims who attend religious ceremonies, especially during Ramadan."

The Islamic Republic of Iran has a population of 83 million, 90 percent of whom are