Injured people are treated at Amai Pakpak Medical Center in Marawi, Lanao del sur province, after a bomb attack during a Catholic mass at Mindanao State University's gymnasium. (PHOTO / AFP)

MANILA - An explosion occurred inside a gym of Mindanao State University in Marawi City, Lanao del Sur province in the southern Philippines on Sunday morning, killing at least four and injuring 50 others, local military and police said.

The Mindanao State University has decided to suspend classes until further notice 

The blast occurred past 7 am local time when students and teachers gathered for a Catholic mass inside the gym, according to local police.

Major General Gabriel Viray, the commander of the Army's 1st Infantry Division, said the deaths include three females and a male.

However, local media GMA Network put the death toll at 11, citing a local hospital source.

Police said the blast may have been caused by an improvised explosive device.

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Medical workers treat an injured person at Amai Pakpak Medical Center in Marawi, Lanao del sur province, after a bomb attack during a Catholic mass at Mindanao State University's gymnasium. (PHOTO / AFP)

Philippine President Ferdinand Romualdez Marcos denounced the fatal bombing, which he blamed on "foreign terrorists."

Defense Secretary Gilbert Teodoro told a press conference Sunday afternoon that "there are strong indications of a foreign element" in the attack.

Armed Forces of the Philippines (AFP) Chief of Staff General Romeo Brawner said the military "is continuously gathering intelligence operations so that we will be able to run after the perpetrators."

The Mindanao State University condemned "this senseless and horrific act," saying it was "deeply saddened by and appalled by the act of violence."

The university has decided to suspend classes until further notice and deployed additional security personnel to safeguard the campus in the capital and the largest city of Lanao del Sur on Mindanao island.

In 2017, local terrorists pledging support to the Islamic State, including a faction of the Abu Sayyaf Group, the Maute Group, among others, occupied and held the lakeside city for five months, resulting in over 1,200 deaths, hundreds of thousands of residents displaced, and many parts of the city destroyed.