The death toll from an overnight suicide attack on a Shi'ite Muslim procession in Pakistan's biggest city rose Tuesday to 33, in violence blamed on extremists trying to hamper the fight against militants by sparking a sectarian war. Angered over Monday's attack, Shi'ites set fire to buildings and dozens of vehicles in Karachi, a sign of frustration by the minority sect, which has suffered frequent attacks by Sunni extremist groups who regard them as heretical. TV footage showed firefighters struggling to extinguish blazes Tuesday. Sagheer Ahmed, the provincial health minister, said the death toll from the attack had risen to 33. Pakistani television stations said that three people who were hospitalized died overnight of their injuries. Pakistani authorities say sectarian groups have teamed up with Taliban and al-Qaida 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. 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 sectarian, ethnic and political violence. The suicide bomber who struck Monday targeted thousands of Shi'ites marching through the streets to observe Ashoura, the most important day of a monthlong mourning period for the seventh-century death of the Prophet Muhammad's grandson, Imam Hussein. "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. "I saw walls stained with blood and splashed with human flesh." Residents in apartments near the blast site tossed down body parts that had been cast into their homes from the explosion, while birds dove down to pick at the flesh amid damaged vehicles and motorbikes. Authorities found the intact head and torso of the suicide bomber on the third floor of a nearby office building, where it had crashed through a window, said bomb disposal squad official Munir Sheikh. Some 35 pounds (16 kilograms) of high explosive were used in the bombing, he said. No group claimed responsibility for Monday's attack, but Pakistani Interior Minister Rehman Malik pointed his finger at a cluster of militant groups, including the Pakistani Taliban, al-Qaida, Lashkar-e-Jhangvi and Jaish-e-Mohammad, that he said have a joint goal to destabilize Pakistan. "These are people who are against democracy, against our religion, against our Pakistan," said Malik. Malik appealed to the Shi'ite community to cancel processions for the next two days. 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 Shi'ites torching at least three vehicles. On Saturday, another blast near a Shi'ite procession wounded 19 people. Authorities attributed that explosion to a firecracker that was so powerful it left a crater in the road. A suicide bomber struck a Shi'ite procession Sunday in Muzaffarabad, the capital of Pakistan-controlled Kashmir, killing eight people and wounding another 80. The bombing was a rare sectarian attack in an area police say has little history of militant violence.