KARACHI - AT LEAST 21 people have been killed in a fresh bout of political violence ahead of a key by-election in Pakistan's southern port city of Karachi, a senior police official said on Sunday.

The unrest follows a deadly wave of violence in the metropolis that claimed 85 lives in the wake of the death of lawmaker Raza Haider, who was shot dead by gunmen in August.

The targeted killings come as the city heads to the polls on Sunday to vote in a replacement for Haider, a Muttahida Qaumi Movement (MQM) lawmaker, to a seat in the provincial assembly in the teeming city of 16 million.

'Twenty-one people have been killed in targeted killings in western and southern neighbourhoods of Karachi since Saturday,' city police chief Fayyaz Leghari told AFP.

'At least 17 people were killed Saturday and four more people died Sunday after gunmen opened fire on a passenger bus in Banaras town,' Leghari added.

He said the police had arrested 20 suspects during raids and were continuing to investigate.

Tensions are high between MQM and the Awami National Party (ANP), which each represent different communities in Karachi, straddling political fault lines in the city.

Violence kills 21 in Karachi