A suicide bomber struck a procession marking a Shia Muslim holy day in Pakistan's biggest city today, killing 30 people in an attack blamed on extremists trying to spark a sectarian war. Outraged Shias responded to one of the worst sectarian attacks in the country this year by setting fire to buildings and vehicles at the blast site and pelting security forces with stones. The minority sect has suffered frequent attacks by Sunni extremist groups.

Authorities said these sectarian groups have teamed up with Taliban and al-Qaida militants waging war against the government in a joint effort to destabilise Pakistan. More than 500 people have been killed in attacks since mid-October when the army launched an anti-Taliban offensive in the country's north-west.

"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.

The southern city of Karachi has largely been spared the Taliban-linked violence that has struck much of the rest of the country. But it has been the scene of frequent sectarian, ethnic and political violence. The suicide bomber who struck today targeted thousands of Shias marching through the streets to observe Ashura, the most important day of a month-long mourning period for the seventh-century death of the Prophet Muhammad's grandson, Imam Hussein. Naseem Raza, a 26-year-old who was marching in the procession, said: "I saw walls stained with blood and splashed with human flesh."

The Karachi police chief, Waseem Ahmad, said police were investigating the possibility that a second suicide bomber was involved in the attack. The blast wounded 60 people, said Sagheer Ahmad, the health minister in Sindh province, where Karachi is the capital.

Major Aurangzeb Khan, a spokesman for paramilitary troops who were protecting the procession, said the death toll would have been much higher if one of the soldiers had not spotted the suicide bomber and tackled him.

Shias went on a rampage after the blast, smashing shops and setting fire to a market, two other buildings and many vehicles. Some roamed the scene with guns and fired into the air.

No group has claimed responsibility for the attack, but Pakistan's interior minister, Rehman Malik, pointed the 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 destabilise Pakistan. Malik appealed to the Shia community to cancel processions for the next two days "for the sake of security and to save precious lives".

Today'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. On Saturday, a blast near a Shia procession wounded 19 people.