More than 70 people have been killed in Pakistani coastal city Karachi this week, following political tensions leading up to a local by-election.

The by-election was called to fill a position left vacant after Raza Haider, a politician in Karachi's dominant Muttahida Qaumi Movement (MQM), was killed two months ago, sparking widespread violence. The party is facing a challenge from the ANP, which is mainly supported by the city's ethnic Pashtun population.

Karachi has a long history of sectarian violence and was a main target of al Qaeda-linked fighters after the September 11, 2001, attacks on the US, when Pakistan joined the US-led campaign against such groups.

Al Jazeera's Zeina Khodr finds out how political parties have been blamed for the recent violence in Pakistan's largest city.