BEIRUT — Millions of Shiite Muslims across the world are commemorating Ashura on Friday and Saturday, at a time of heightened tensions between Muslim countries and the West.

Ashura falls on the 10th day of the Muslim month of Muharram. On that day, Shiites mark the anniversary of the killing of Imam Hussein, the Prophet Muhammad’s grandson, during the Battle of Karbala in Iraq in the seventh century. The death of Hussein, whom Shiites view as a martyr killed by Sunnis, is one of the many reasons behind the deep rift between Sunnis and Shiites.

In the past years, attacks against Shiites marking Ashura have been frequent. On Thursday, a car bomb in the Syrian capital, Damascus, exploded, killing at least six people and wounding more than 20 others. The explosion occurred near the Sayeda Zeinab shrine, one of Syria’s most visited Shiite shrines. It came days after a bomb planted in a motorcycle exploded also near the shrine on Tuesday, wounding two civilians, according to state TV. The attacks came despite heavy security deployed in the area ahead of Ashura, which Syrian Shiites will mark on Saturday.