The bomb exploded outside the Immaculate Conception cathedral in Cotabato city as churchgoers walked out after attending Mass. Two passers-by were killed instantly in the attack and a third died on the way to a hospital, said regional military commander Maj. Gen. Alfredo Cayton.

The improvised explosive was hidden near a row of food stalls selling roasted pig, Cayton said.

Cathedral guard Nestor Luna said shrapnel was sent flying in all directions. Luna was wounded in the head, arm and feet and said he thought his time had come.

"I wanted to help the wounded but I felt dizzy and saw my white uniform soaked in blood. Somebody helped me walk away," Luna told The Associated Press by telephone from the Cotabato Regional and Medical Center.

"Whoever did this should know that they hit so many innocent civilians," he said.