Ashoura is marked on the tenth day of Muharram, the first month of the Islamic calendar, by all Muslims.
It marks the day Nuh (Noah) left the Ark and the day Musa (Moses) was saved from the Pharaoh of Egypt by God.
The Prophet Muhammad used to fast on Ashoura in Mecca, where it became a common tradition for the early Muslims.
They were called Shia, which means "the supporters" in Arabic.
Ali was murdered in AD 661 and his leading opponent Muawiya bin Abi Sufiyan became caliph.
Muawiya was succeeded by his son Yazid. Ali's son al-Hussein refused to accept Yazid's legitimacy and fighting between the two broke out.
Al-Hussein, accompanied by his family and a few fighters, faced Yazid's army.
A defiant al-Hussein refused to surrender even though he realised he was outnumbered.
After a brief battle, he and his followers were killed.
Shia festival
The primary rituals and observances on Ashoura consist of public expressions of mourning. Some in the Shia community resort to flagellating themselves with chains and the blunt ends of swords. This is intended to exemplify the suffering Imam al-Hussein experienced shortly before his beheading.
The death of Imam al-Hussein is considered by the Shia community as a symbol of humanity's struggle against injustice, tyranny and oppression.
However, in recent years, some Shia clerics have been discouraging the bloodletting, saying it creates a backward and negative image of their community. Such leaders encourage people to donate blood to patients in need.