The explosion happened Sunday morning after Cotabato Archbishop Orlando Quevedo was about to start his homily during the early morning mass at the Immaculate Concepcion Cathedral.
About 35 people were rush to nearby hospitals for medical treatment including a soldier, bystanders and students.
The explosion was so loud as if the cathedral was to collapse,” said Merly Sandoval, a churchgoer.
“People were running toward the altar after the blast…It was like a very loud thunderstorm and the sound reverberated inside the jam-packed cathedral,” said another churchgoer Isabel Joven.
"Everybody was screaming. We saw blood coming out of those lying on the ground near the entrance of the cathedral compound," Sandoval said.
A suspect who was seen and presumed to have planted the bomb was apprehended and is undergoing interrogation.
Military authorities in the region quickly blamed the Moro Islamic Liberation Front (MILF) for the bomb explosion.
"The special operations group of the MILF is behind this," Ponce said, adding that the MILF had carried out similar attacks in recent weeks.
The MILF has been fighting government troops in retaliation for the proposed deal with government that would give them control of ancestral lands in southern Philippines which was rejected by the Philippine Supreme Court.