MANILA — Four people have been killed in an explosion at a Catholic Mass in the southern Philippines on Sunday morning.

The incident occurred at the gymnasium of Mindanao State University in Marawi, the country’s largest Muslim city.

Forty-two others suffered mostly minor wounds, authorities said, adding the situation was “under control”.

In 2017, Marawi was the scene of a five-month battle between government forces and militants with links to the Daesh (so-called IS) group.

The Daulah Islamiyah-Maute Group could be behind Sunday’s bombing, said Brig. Gen. Allan Nobleza, the police commander in the region.

Gen. Nobleza said 11 of its members died in an encounter with the Philippine Army last Friday in neighboring Datu Hoffer Ampatuan town — suggesting Sunday’s explosion could be a form of retaliation.

President Ferdinand Marcos Jr condemned the blast as a “senseless and most heinous” act, which was “perpetrated by foreign terrorists”. He did not elaborate.

He appealed to the public to remain calm. “Rest assured, we will bring the perpetrators of this ruthless act to justice,” he said.

A grenade or an improvised bomb is likely to have caused the explosion, officials said, citing a preliminary investigation.

Photos shared on social media by local officials showed plastic chairs in disarray and dark fragments on the ground of the MSU gymnasium after the blast. Aside from its floor, the building did not appear to sustain major damage.

Those who were brought to a nearby hospital received treatment mostly for minor wounds and bruises, photos suggested.

The Provincial Governor, Mamintal Adiong Jr, said many of the three dozen wounded have been sent home.

Masses on Sunday drew larger crowds than usual across the Philippines as it is the start of Advent, the Catholic Church’s four-week vigil to Christmas Day.

Nearly 80% of the country’s 113 million population are Catholic and it is not uncommon for school gymnasiums and even shopping malls to designate areas for Sunday Mass, especially in places where there are no churches.

The MSU, one of country’s largest universities, said it was “deeply saddened and appalled” by the “senseless and horrific” violence.

“Violence has no place in a civilized society, and it is particularly abhorrent in an institution of higher learning like MSU,” it said.

“We stand in solidarity with our Christian community and all those affected by this tragedy.”

The university added that additional security staff had been deployed on its campus and that all academic activities would be suspended until further notice.

Mindanao is home to the country’s Muslim minority and has borne the brunt of insurgency and extremist violence.

In 2012, Manila and the country’s largest Muslim rebel group agreed to establish an autonomous region in Mindanao and the first elections for the regional assembly was held in 2022. However, sporadic violence flares up from time to time. — BBC