KARACHI (AFP) - Gunmen on Friday shot dead a Pakistani lawmaker, his son and a passer-by outside a mosque in the country's business capital of Karachi, officials said.

Sajid Qureshi, in his early 50s, and his 25-year-old son were targeted in a drive-by shooting as they left a mosque after attending Friday prayers in congested Nazim Abad neighbourhood.

"Gunmen on a motorcycle fired at (Sindh) provincial assembly member Sajid Qureshi, his son and a pedestrian when they were coming out of mosque," senior police official Amir Farooqi told AFP.

Mr Qureshi was a member of the secular Muttahida Qaumi Movement (MQM), the most powerful political party in Karachi now considering whether to join the government in southern Sindh province.

"He died on the spot, while his son and the third victim died at hospital," Mr Farooqi said.

Nasir Jamal, a senior member of MQM, confirmed the death of Qureshi and his son.

MQM has long held sway in Karachi, a port city on the Arabian Sea, and its leader-in-exile Altaf Hussain lives in London.

Karachi, a city of 18 million people, contributes 42 per cent of Pakistan's GDP but is rife with murder and kidnappings and has been plagued for years by ethnic, sectarian and political violence.