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Mr. Mirza apologized for his comments Thursday, calling members of the Urdu-speaking community his “brothers.” His mea culpa came as thousands of people rallied in the center of the city to condemn him and burn his effigies.

Mr. Hussain, who lives in self-imposed exile in London, appealed to his supporters to call off protests, a move that seemed to help calm the situation by Thursday evening.

A large number of MQM’s supporters are Urdu-speaking descendants of people who came to Karachi from India soon after the birth of Pakistan in 1947. The party dominates politics in urban areas of Sindh, including Karachi, but over time it has seen challenges to its power from the People’s Party and the Awami National Party, a Pashtun nationalist party.

Ashraf Khan reported from Karachi.