At least 27 people were killed and 77 others injured on Sunday in twin explosions during Mass at a church in Philippines' Sulu province, police said.
A senior police officer told the media that the blasts hit the Jolo Cathedral at 8.15 a.m., reports Xinhua news agency.
Philippine National Police (PNP) Director General Oscar Albayalde said the explosions were caused by improvised explosive devices (IEDs).
He said one went off inside the church while the second one near its entrance.
"It went off one minute apart," Albayalde said.
"We strongly condemn this atrocious act of taking the human lives for whatever purpose the perpetrators may have. The (PNP) together with other government security forces will make sure those who are behind this will be soon identified and put behind bars," he added.
No group has claimed responsibility for the attack that comes after the holding of a referendum on the creation of a Muslim autonomous region in the south of the country, called Bangsamoro, and conceived as a peaceful solution to decades of separatist conflict provoked by Islamist radicals.
The province of Sulu - whose capital is Jolo - voted against integrating into Bangsamoro, but since it is part of the Autonomous Region of Muslim Mindanao (ARMM) with four other provinces its votes are counted in a block and will belong to this new entity.
In Sulu, several radical groups linked to the Islamic State, such as Abu Sayyaf or the Maute group, have been responsible for bloody attacks in the past.