BAGHDAD — Insurgents mounted another attack against Iraq’s Shiites on Saturday, as an explosion in the southern city of Basra ripped through a group of pilgrims headed to a mosque to commemorate one of the holiest Shiite holidays.

The explosion hit a tent where pilgrims were being fed around 8:30 a.m., killing at least 53 people, including several police officers at a nearby checkpoint, and wounding more than 130 others, local officials said.

There were conflicting reports, though, about the cause of the blast. Some officials said it was a roadside bomb, while others said a suicide bomber had attacked the crowd.

The pilgrims were traveling to a mosque in the city of Zubayr, just west of Basra, for the last day of Arbaeen, the solemn holiday at the end of the 40-day mourning period for the death of Imam Hussein ibn Ali, a grandson of the Prophet Muhammad.