An explosion at a Catholic Mass in a university gymnasium in Marawi city in the Philippines killed four people and injured dozens more on Sunday.

What do we know so far?

The explosion occurred at Mindanao State University in the Lanao del Sur province in the southern part of the country.  

The blast happened during a regular service at Mindanao State University's gymnasium in Marawi, the country's largest Muslim city, regional police Chief Allan Nobleza said.

"We're investigating if it's an IED (improvised explosive device) or grenade throwing," he said.

Fragments of a 16-mm mortar were recovered at the scene, senior police official Emmanuel Peralta told a news conference later.

"It was really sudden and everyone ran. When I looked behind me people were lying on the floor. We didn't know what happened because everything happened so fast," university student Chris Jurado told AFP news agency.

He said the incident happened during the first Bible reading of the morning mass at 7:00 am (2300 GMT Saturday).

Classes were suspended and more forces were deployed across the university.

The so-called "Islamic State" (IS) group claimed responsibility for the attack, saying on Telegram that its members had detonated the bomb.

Defense Secretary Gilberto Teodoro said there were "strong indications of a foreign element" in the bombing.

President Ferdinand Marcos Jr condemned the bombing on X, formerly Twitter, labeling the attack "the senseless and most heinous acts perpetrated by foreign terrorists. Extremists who wield violence against the innocent will always be regarded as enemies to our society."

Local officials 'stand in solidarity' with Christian community

"I condemn the violent bombing incident that transpired this morning. Terroristic attacks on educational institutions must also be condemned because these are places that promote the culture of peace," Lanao del Sur Governor Mamintal Adiong, Jr., said in a statement. 

The Lanao del Sur provincial government's Facebook page showed him visiting "wounded victims of the bombing" at a medical facility.

"We unequivocally condemn in the strongest possible terms this senseless and horrific act," Mindanao State University said in a statement on Facebook. "We stand in solidarity with our Christian community and all those affected by this tragedy," it said. 

Marawi is the country's largest Muslim city and was besieged by IS militants for five months in 2017.

The Philippine military said on Saturday they had killed 11 militants, including members of the Dawlah Islamiyah-Philippines, a pro-IS group, in a military operation the day before in Maguindanao del Sur province.

mm, tg/sms (AP, Reuters)