LAHORE, Pakistan—Karachi, Pakistan's biggest city that has long been plagued by ethnic and political tension, saw some of the worst violence and irregularities in Saturday's election.

A bomb blast at the office of a secular contender, the Awami National Party, killed at least 11 people, according to police and party officials. The ANP has been targeted by the Pakistani Taliban during the election campaign, along with the other two major secular political parties.

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