Iranian Shiite Muslims on Tuesday commemorated Ashoura, a remembrance of the 7th century martyrdom of the Prophet Muhammad’s grandson, Hussein, that gave birth to their faith.

More than 1,340 years after Hussein’s death, Tehran and other cities across the country were adorned with symbols of Shiite piety and repentance. Red flags represented Hussein’s blood, black funeral tents and clothes indicated mourning, and processions of chest-beating and self-flagellating men expressed fervor. Some sprayed water over the mourners in the intense heat.

Iranian state TV reported that 6 million Iranian pilgrims traveled to the Iraqi city of Karbala, where Hussein is entombed in a gold-domed shrine, and broadcast live images of the procession.

Many Shiite Muslims across the Middle East commemorated Ashoura on Tuesday.

In the Omani capital of Muscat, a shooting at a Shiite mosque killed four people and wounded dozens on Tuesday. Iran’s foreign ministry condemned the attack.

Shiites represent more than 10% of the world’s 1.8 billion Muslims and view Hussein as the rightful successor to the Prophet Muhammad. Hussein’s death in battle at the hands of Sunnis at Karbala, south of Baghdad, ingrained a deep rift in Islam and continues to play a key role in shaping Shiite identity.