Police stopped a number of Egyptian Shias from entering the Hussein mosque in Cairo Saturday afternoon, to celebrate the day of Ashura, state-run newspaper Al-Ahram said on its website.

A force from the Gamaliya Police Department was stationed on Friday in front of the main door of the mosque to inspect worshipers before prayers, and security forces prevented a number of Shias from entering and ordered them to perform afternoon prayers at other mosques nearby. Mosque management also closed the Imam Hussein shrine.

The move came after Walid Ismail, chairman of a Sunni Muslim group, filed a complaint with the police demanding they prevent any Shia rituals inside the mosque honoring the martyrdom of Hussein.

The Endowments Ministry said it supported the minister’s decision to prevent celebrations inside mosques, and Al-Azhar issued a statement warning against celebrations on Ashura. It added that the day was for fasting and worship, and it stressed Egypt’s Sunni identify and condemned what it described as “heretical” practices in celebrating Ashura.

Ashura is the day where Muslims mourns the death of Imam Hussein, the grandson of the Prophet Mohamed who was killed in 680 AD in Karbala, Iraq, by the armies of the caliph Yazid. The day is particularly significant for Shia Muslims, who gather in large numbers and sometimes beat their chest as a sign of mourning in honor of Hussein. The Hussein Mosque is believed by many worshippers to contain Hussein’s head.

Sunnis recognize the first four caliphs as the Prophet Mohamed’s successors, while Shias believe that he named his son-in-law Ali as his true successor. The dispute has continued up until today, and the ongoing Sunni-Shia split has fueled instability and strife in some countries in the region.