A motorcycle bomb has killed left at least six people and wounded dozen more in the Pakistani city of Karachi.

Tuesday's blast apparently targeted buses transporting activists after a joint rally organised by the Muttahida Qaumi Movement, the city's dominant political party, and Tehreek-e-Minhajul Quran - the movement to follow the path of the Quran - led by Muslim scholar Tahir-ul Qadri.

"The bomb was planted in a motorcycle," Asif Ijaz, a senior police official, told the AFP news agency.

Imran Shokat, a spokesman for police in southern Sindh province of which Karachi is the capital, confirmed the incident and added that the motorbike was parked in the congested Aisha Manzil neighbourhood of the city.

Karachi, the commercial capital of Pakistan with an estimated population of 18 million, is in the grip of a long-running wave of political and sectarian violence.

The city's port is used by the US and NATO to ship supplies to the war in neighbouring, landlocked Afghanistan.

Source: Agencies