Eight more people were wounded in the attack, said Aftab Chanur, an official at the hospital who gave the death toll.
The four gunmen on motorcycles first lobbed grenades at the building where a Sufi cleric was receiving his followers, then raked it with automatic fire, said police official Javed Odho. He said women and children were among the dead and wounded.
Pakistan is 95 percent Muslim, and the majority practice Sufi-influenced Islam.
But their shrines and followers have come under attack in Pakistan by Sunni Muslim militants who dont consider them to be true Muslims.
No one claimed responsibility for the attack. But suspicion is likely to fall on militants such as the Pakistani Taliban or their affiliated sectarian groups who follow a strict interpretation of Islam that considers many other Muslims such as Sufis or minority Shiites to be heretics. In recent years militants have often targeted shrines, which they consider to be sacrilegious.
In January, militants killed six people at the shrine of a Sufi saint in Karachi. After that attack, militants also threatened the cleric who was targeted Sunday, telling him he should close down the house of worship where he would receive his followers, said Odho, the police official.
Karachi is a fast-growing city in southern Pakistan with an estimated 18 million residents that has often been plagued by political, religious and sectarian violence.
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