Pakistani paramilitary troops patrolled the deserted streets of the southern city of Karachi yesterday to stop any fresh violence after a suicide bomb blast at a Sunni Muslim prayer meeting killed at least 57 people.

Tuesday?s strike by two suspected suicide bombers was the worst ever on Karachi, which has been plagued by sectarian violence and Islamist militant organisations angered by President Pervez Musharraf?s support for the U.S.-led war on terrorism.

?There are also reports that some people are still missing,? Salahuddin Haider, spokesman of the provincial Sindh government, said. ?Our initial investigations suggest that there were at least two suicide bombers involved in the attack. We have found the body parts, including the heads, of the suspected attackers.?

Nobody had claimed responsibility for the attack and a Pakistani official said authorities were struggling to identify the group behind the blasts.

?Jihadis are all out against President Musharraf, so they may be involved,? said Salahuddin Haider, spokesman of the provincial Sindh government.

Aside from homegrown Islamist groups, some of which have forged ties with al Qaeda, Haider said ethnic Baluch rebels from southwest Pakistan could also be on the list of suspects. Indian and Afghan involvement could not be ruled out, although there was no evidence, he added.

Islamist militants, however, are more likely to use suicide bombers than groups driven by secular political motives, raising chances it was a sectarian attack.

The attackers struck while worshippers from a Sunni Muslim organisation, Jamaat-e-Ahle Sunnat, had gathered for prayers at the end of a day that marked the anniversary of Prophet Mohammad?s birth. ?It was a very well-planned and co-ordinated attack, and apparently it seems that the attackers were very well-trained,? said Haider, adding that they struck in a way to inflict maximum damage.

Among the dead were several locally well-known leaders of Jamaat-e-Ahle Sunnat, which has hundreds of thousands of followers among Pakistan?s dominant Barelvi sect of Sunni Muslims, raising fears of a violent backlash.

Leaders of Sunni Tehrik, a major Barelvi Muslim group, threatened to call for protests if the culprits were not arrested.

?We have given a 48-hour ultimatum to the central government to arrest those who plotted this attack and hang them publicly,? Shahid Ghauri, a central leader of Sunni Tehrik told a news conference. ?The Sunni people are waiting for our orders.?

The Sindh government announced a three-day mourning period, and closed down all educational institutions. Around 3,000 mourners, many of them wearing green turbans, attended the funeral of a Sunni leader, Hanif Billu, which like others passed off peacefully.

But the city was tense and deserted, with police and paramilitary troops patrolling the streets shorn of public transport. Almost all filling stations were closed, covered with thick clothes and protected by barriers to avoid any attack.

Dozens of youths wandered around the bomb site, gaping at the badly damaged stage and the hundreds of shoes left behind by dead, wounded and panicked worshippers. There were two police vans parked at the scene.

Karachi has been one of the main battlegrounds for sectarian violence between Pakistan?s Sunni Muslim majority and minority Shi?ite Muslims over the past two decades.

Officials said the government had asked foreign franchises such as the KFC and McDonald?s fast food outlets to remain closed as they are often targeted at such times.

The provincial government has also asked the army stationed in the city to be ready to deploy troops at short notice, according to military officials. No violence was reported during the first funerams, but those of the Sunni group?s leaders were still to be held.