Tens of thousands of Muslims across the globe on Tuesday observed Ashura, an annual commemoration mourning the seventh-century death of Prophet Muhammad’s grandson Imam Hussein.

Ashura is marked on the 10th day of Muharram, the first month of the Islamic calendar, by all Muslims.

It is a particularly important period of mourning for Shia Muslims, as it marks the anniversary of the seventh-century Battle of Karbala in present-day Iraq, when Hussein was killed.

To mourn his death in the year 680, Shia worshippers, wearing black, cry and beat their chests in unison. Some also flagellate themselves with swords and knife-edged chains.

More than 1,340 years after Hussein’s death, Baghdad, Tehran and other major cities were adorned on Tuesday with symbols of Shia piety and repentance: red flags for Hussein’s blood, symbolic black funeral tents and black dress for mourning, and processions of men and boys expressing fervour in the ritual of chest beating and self-flagellation with chains.

The death of Hussein is considered by the Shia community as a symbol of humanity’s struggle against injustice, tyranny and oppression.

The primary rituals and observances on Ashura consist of public expressions of mourning. Sunni Muslims commemorate the day through voluntary fasting.

In parts of the world including Pakistan, India and Bangladesh, Ashura will be marked on Wednesday.

In the Omani capital of Muscat, a shooting at a Shia mosque killed at least six people and wounded dozens late on Monday.