KARACHI, Pakistan - Authorities appealed for calm yesterday after a bombing against a Shiite Muslim procession killed 43 in Pakistan's largest city of Karachi, setting off riots and igniting fears of sectarian unrest.

Security was tight in the city of about 18 million as thousands of people gathered in central Karachi for funerals of some of those killed in Monday's bombing of a Shiite procession marking the key holy day of Ashoura.

The attack sparked riots as people rampaged through the city, setting fire to markets and stores, including the port city's largest wholesale market. More than 200 firefighters were still battling the flames 24 hours after the attack, with authorities calling for reinforcements from the city of Hyderabad, 105 miles north of Karachi, Pakistan's main commercial hub.

Two buildings with dozens of shops and offices had collapsed and two more were in danger of falling, said the Karachi fire chief, Ehteshamul Haq.

Pakistani officials blamed unnamed "miscreants" for the violence after the attack rather than Shiites involved in the procession, possibly in an attempt to temper sectarian tensions.

"We believe that it was a planned conspiracy," said Interior Minister Rehman Malik. "Those who went violent were miscreants and not the protesters."

It remained unclear who was behind Monday's bombing, at the start of a procession of Shiites marking Ashoura, the most important day of a monthlong mourning period for the seventh-century death of the Prophet Muhammad's grandson Imam Hussein. Minority Shiites in Pakistan have suffered frequent attacks by Sunni extremist groups that regard them as heretical.

Karachi has largely been spared the Taliban-linked violence that has struck much of the rest of the country, a fact that analysts believe is driven by the group's tendency to use the teeming metropolis as a place to rest and raise money. But the city has been the scene of frequent ethnic, political, and sectarian violence.

Bomb disposal squad official Munir Sheikh said that about 35 pounds of high explosive were used in the bombing.

Senior health official Hashim Raza said the death toll increased to 43 yesterday. Many among the dozens wounded were critically hurt, and several died overnight and yesterday morning.

No group claimed responsibility for the attack, but Malik on Monday pointed his finger at a cluster of extremist groups, including the Pakistani Taliban, al-Qaeda, Lashkar-e-Jhangvi, and Jaish-e-Mohammad, that he said had a joint goal to destabilize Pakistan.