A suicide bomber on Monday struck Pakistan’s largest procession of Shia Muslims on the holiest day in their calendar, killing at least 27 people and wounding dozens more, defying a major security clampdown.

The blast unleashed pandemonium at M A Jinnah Road, one of the biggest boulevards in Karachi, where angry mourners threw stones and opened fire into the air, sparking appeals from the authorities for calm.

Tens of thousands of police and paramilitary forces had been deployed, fearing sectarian clashes or militant bombings would target the Shia faithful who whip themselves to mourn the seventh-century killing of Imam Hussein.

“It was a suicide attack. He was walking with the procession and he blew himself up,” Interior Minister Rehman Malik told a private television, appealing on the Shia community to suspend their commemorations.