By Imtiaz Shah

KARACHI, Apr 14 (Reuters) Sporadic trouble erupted in the Pakistani city of Karachi today and traders kept businesses shut after Sunni Muslim groups called for a strike to protest against the killing this week of 57 people at a prayer meeting.

A suicide bomber targeted a congregation of more than 15,000 Sunni Muslims, Pakistan's majority sect, celebrating the birthday of the Prophet Mohammad at a city park on Tuesday.

''The city is closed and there were reports of some minor skirmishes,'' said Salahuddin Haider, spokesman for the government of Sindh province.

''But no major incidents of violence have been reported.'' No group has claimed responsibility for Tuesday's attack and investigators have still to narrow down a list of suspects that includes militant groups from sectarian, separatist and ethnic groups, as well as al Qaeda-linked Islamists.

The attack on the Sunni community was the worst ever in Karachi, Pakistan's largest city and its commercial capital.

Buses stayed off the streets after angry young men torched at least three vehicles overnight. They burned tyres in some neighbourhoods and threw stones at vehicles to force people to heed the strike call.

The Jamaat-e-Ahle Sunnat, a religious group of the dominant Barelvi Sunni Muslim sect, which organised the Tuesday gathering, called for a countrywide strike to mourn the dead.

Streets in many parts of Karachi were deserted and the situation in many other parts of Sindh province was the same, residents said.

Police and other security forces stepped up patrols in parts of the city, including in the vicinity of mosques, which were due to hold weekly Friday prayers.

''We have deployed an extra 2,500 policemen, in addition to the 5,000 normally on duty, and have increased patrols,'' said senior police official Mushtaq Shah.

The Karachi stock exchange functioned normally, with the KSE 100 share index up 0.33 per cent at 1320 hrs IST.

PHOTOGRAPH Police released a photograph of the bearded, bloodied head of a suspected suicide bomber yesterday and said a DNA test would be carried out in the hope of identifying the attacker.

Suicide bombing has become more common in Pakistan.

Militants, angered by President Pervez Musharraf's support for the US-led war on terrorism, have used suicide bombers in attempts to kill both the president and his prime minister.

Police were also questioning a wounded Shi'ite Muslim, and looking for an Afghan he said had accompanied him to the park on Tuesday.

Several other people had been ''detained for debriefing'', said Interior Ministry Secretary Syed Kamal Shah.

''We are pursuing investigations very rigorously but as such, there has been no headway so far,'' Shah said.

REUTERS KD RS1419