The attack is the third in three days in Karachi, Pakistan's most populous city. It targeted the minority Shiites as they were commemorating Ashura, a major event on the Shiite calendar. A suicide bomber blew himself up today in the middle of a Shiite Muslim procession in Karachi, the commercial capital of Pakistan, killing at least 20 people, wounding dozens more, and heaping further pressure on the troubled nation's already beleaguered government.