As the suicide bomber clutched the detonator to his explosive belt, preparing to spray fire and shrapnel into a religious procession here, an Iraqi police officer named Bilal Ali Muhammad faced a choice between his own life and something larger.

If he ran and took cover, Mr. Muhammad, 31, had a chance to save himself, to continue supporting his widowed mother, to help put his younger brother through college and to watch his three young daughters grow up.

Instead, the officer — a Sunni Muslim — threw himself onto the bomber, blunting the explosion’s impact on the Shiite worshipers.