Syed Manzar Imam, a member of the liberal Muttahida Qaumi Movement and three of his guards were shot to death Thursday by members of Tehrik-i-Taliban Pakistan, which claimed responsibility, Dawn News reported.
News of the death of the 46-year-old lawmaker set off violence in Karachi and other cities. In Karachi, four men were killed and several other people were injured.
The MQM condemned the killing and vowed party members would not be intimidated.
"Our peacefulness should not be taken as our weakness," MQM legislator Raza Haroon told the media after Imam's funeral.
He urged other political parties to set aside their differences and work to rid Pakistan of terrorism.
Businesses, markets and schools in Karachi and Hyderabad remained closed in observance of the mourning period. The head of the transportation association said all public transport vehicles would stay off the roads.