Angry mourners attack a police car after a bomb blast struck a procession in Karachi, Pakistan on Monday. (Shakil Adil/Associated Press)

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

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

The attack sparked riots as people rampaged through the city, setting fire to markets and stores. Firefighters were still battling the flames Tuesday, with authorities calling for reinforcements from the city of Hyderabad, 170 kilometres north of Karachi, Pakistan's main commercial hub.

Karachi Mayor Mustafa Kamal said the city's largest wholesale market was on fire, and that hundreds of shops had been destroyed, with damages estimated to run into millions of dollars.

Interior Minister Rehman Malik, who visited Karachi on Tuesday, said authorities were still trying to determine whether the attack had been carried out by a suicide bomber, as he had said Monday.

"The investigation is still going on to determine whether it was a suicide attack or some improvised explosive device was used," said Malik. "If anyone is trying to cripple Karachi, then he is also trying to cripple Pakistan."

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

It was unclear who was behind Monday's bombing. Pakistani authorities say sectarian groups have teamed up with Taliban and al-Qaeda militants waging war against the government in a joint effort to destabilize Pakistan.

More than 500 people have been killed in attacks since mid-October when the army launched a major anti-Taliban offensive in the country's northwest.

"A deliberate attempt seems to be afoot by the extremists to turn the fight against militants into a sectarian clash and make the people fight against one another," said President Asif Ali Zardari in a statement Monday.

"I fell down when the bomb went off with a big bang," said Naseem Raza, a 26-year-old who was marching in the procession when the blast happened. "I saw walls stained with blood and splashed with human flesh."

Monday's bombing was the third explosion in as many days to hit Karachi, although authorities attributed a blast that wounded 30 on Sunday to a buildup of gas in a sewage pipe. Protests broke out after that blast too, with rioters torching at least three vehicles.

On Saturday, another blast near a procession wounded 19 people. Authorities attributed that explosion to a firecracker that was so powerful it left a crater in the road.

Security was increased across Pakistan for Ashoura, which is the 10th day of the holy month of Muharram, a month of mourning commemorating the seventh-century death of the Prophet Muhammad's grandson.