Shiites in Iraq and Lebanon chanted, paraded, and beat their chests on Tuesday as they marked Ashoura, one of the most important dates on the religious calendar, commemorating the 7th-century martyrdom of the Prophet Muhammad’s grandson Hussein.

The symbols of Shiite piety and penitence blanketed major cities in Iraq, where Hussein was believed killed at the battle of Karbala, south of Baghdad, in 680 A.D.

Every year, hundreds of thousands of people converge on Karbala, some 80 kilometers (50 miles) south of Baghdad, to observe the solemn holy day.

Shiites see Hussein and his descendants as the rightful heirs to the prophet. His killing at the hands of a rival Muslim faction embodies the rift between the Sunni and Shiite sects of Islam and continues to shape the identity of the minority branch of Islam today.

The public rituals of Ashoura often fuel sectarian tensions in places like Iraq, Lebanon, and Pakistan where Islam’s two main sects both reside.

Security forces were on high alert for any violence, as extremist groups that consider the Shiites heretics have seized on the occasion to mount attacks in years past.

In Iraq, the powerful cleric Muqtada Al-Sadr has used the emotional religious occasion to stir up support for his movement, deepening the country’s inter-Shiite divisions. Unable to form a government, Iraq descended further into political chaos last week when thousands of Al-Sadr’s supporters stormed and occupied the parliament building. Their sit-in continues outside the assembly, making it impossible for lawmakers to convene and raising the specter of civil strife.

In the Shiite-dominated Baghdad suburb of Sadr City, Al-Sadr’s portrait hangs from nearly every door. Processions of men and boys expressed extreme fervor in the Ashoura rituals of self-flagellation on Tuesday. They beat their heads and chests in unison and whipped themselves with chains to the point of bleeding.

“We inherited this from our fathers and grandfathers,” said participant Hamza Abdul-Jalil. “God willing, we will continue on this path.”

In Lebanon, processions shut down Shiite areas across the country and Beirut’s biggest suburb.