The December 3 protests in the key port city were prompted by the killing of Maulana Muhammad Ismail, a senior teacher at a Sunni Muslim madrasah, or religious school.
Police say Ismail was shot to death earlier in the day by unknown assailants as he drove from home to the Jamia Arabia Ahsanul Uloom madrasah.
Police are looking for the motive behind the killing.
It is the latest among hundreds of political and sectarian assassinations this year in Pakistan's largest city.
Karachi is plagued by high levels of crime and infighting among ethnic, religious, and political groups.