Witnesses said the first blast inside the Jolo cathedral in the provincial capital sent churchgoers, some of them wounded, stampeding out of the main door. .
Soldiers and police posted outside were rushing in when the second bomb went off about one minute later near the main entrance, causing more deaths and injuries.
Jolo island has long been troubled by the presence of Abu Sayyaf militants, who have carried out bombings, kidnappings and beheadings for many years. .
The initial explosion scattered the wooden pews inside the main hall and blasted window glass panels.
The second bomb hurled human remains and debris across a town square fronting the Cathedral of Our Lady of Mount Carmel, witnesses said.