Military personnel stand guard at the entrance of a gymnasium while police investigators at Mindanao State University in Marawi, Lanao del sur province on Sunday. AFP

At least four people were killed and dozens wounded in a bomb attack on a Catholic mass in the insurgency-plagued southern Philippines on Sunday, with President Ferdinand Marcos blaming "foreign terrorists".

The blast happened during a morning service at Mindanao State University's gymnasium in Marawi, the country's largest Muslim city.

Police Lieutenant General Emmanuel Peralta said four people were killed and around 50 wounded in the blast that was caused by an improvised explosive device. The death toll had earlier stood at three.

Other security officials said the bombing may have been a retaliatory attack for a series of military operations against Islamist militant groups in recent days.

Photos posted on the Lanao del Sur provincial government's Facebook page showed several overturned plastic chairs, shattered glass and debris around a black patch on the floor of the gymnasium.

University student Chris Jurado, 21, told AFP from his hospital bed that the explosion happened during the first Bible reading of the morning mass at 7:00 am (2300 GMT Saturday).

Philippine Defense Secretary Gilbert Teodoro (centre) talks during a press conference in Quezon city. AP

"It was really sudden and everyone ran," Jurado said.

"When I looked behind me people were lying on the floor. We didn't know what happened because everything happened so fast."

Rowena Mae Fernandez, 19, said she did not know what the blast was at first -- then others started running.

"My companion and I also ran, even though we fell on the ground at one point. That was the only thing I remembered until I got out of the gym and I fell again," she said from hospital.

A person inured in a bombing is attended at a hospital in Marawi, Philippines on Sunday. AP

"My friends were crying because they saw my injury."

Marcos condemned the attack by "foreign terrorists", describing it as "senseless" and "heinous".

Mindanao State University issued a statement condemning "the act of violence", as it suspended classes and deployed more security personnel on the campus.

"We stand in solidarity with our Christian community and all those affected by this tragedy," the university said.

Military personnel stand guard at the entrance of a gymnasium on Sunday. AFP

Marawi city Mayor Majul Gandamra urged members of the Muslim and Christian communities to remain united.

"Our city has long been a beacon of peaceful coexistence and harmony, and we will not allow such acts of violence to overshadow our collective commitment to peace and unity," Gandamra said in a statement condemning the attack.

Agence France-Presse