Hangu, Pakistan ? A suicide bomber struck Thursday in Pakistan on the holiest festival for Shiite Muslims, triggering a riot that left a provincial town in flames and at least 27 people dead and more than 50 wounded.

After the bombing, which appeared to be a sectarian attack, security forces battled enraged worshippers who torched shops and cars and took up positions on hills overlooking Hangu, where the sound of gunfire echoed through the smoky streets.

In neighboring Afghanistan, hundreds of Shiites and Sunnis clashed in the western city of Herat, hurling grenades and burning mosques. At least five people were killed and 51 wounded.

The Shiites were marking Ashoura, when they pound their chests and flail their backs with chains and blades to mourn the 7th-century death of Imam Hussain, a grandson of the Prophet Muhammad. Hussain’s death fueled a rivalry between Shiites and Sunnis over who should succeed the prophet.

Sectarian attacks have also often marred the annual rite in Pakistan, but rarely in Afghanistan. Two years ago, a suicide attack on a Shiite procession by Sunni militants in the southwestern Pakistani city of Quetta killed 44 people.

Pakistan declared a curfew and deployed its army to restore order in Hangu, a market town of about 200,000 people.

District police chief Ayub Khan said 23 people died in the bombing and riots that followed. Three other three men and a woman died in a separate shooting on a minibus on the outskirts of town, according to a commander of the paramilitary Frontier Constabulary, Aziz ur-Rahman. Security officials said Shiites protesting the bombing had blocked a road and fired on the vehicle.

Akram Durrani, the top elected official in the province, said a preliminary investigation showed the attack in Hangu was a suicide bombing, but he gave no further details.