A powerful explosion, believed to have been caused by a bomb, tore through a Catholic Mass, killing at least four people and injuring dozens of others in a predominantly Muslim city in the southern Philippines, officials said.

The Sunday morning Mass was under way in a gymnasium at the state-run Mindanao State University in Marawi city when the blast happened, causing panic among dozens of students and teachers and leaving the victims bloodied and sprawled on the ground, said Taha Mandangan, the security chief of the state-run campus.

At least two of the injured are fighting for their lives, he said.

Philippines Deadly Explosion
A Filipino trooper guards the entrance of the site where a bomb exploded in Marawi, southern Philippines (Froilan Gallardo/AP)

“This is clearly an act of terrorism. It’s not a simple feud between two people. A bomb will kill everybody around,” Mr Mandangan told the Associated Press.

Regional military commander Major General Gabriel Viray III said at least four people were killed by the explosion, including three women, and 50 other people were taken to two hospitals for treatment of mostly minor injuries.

Only two of those killed have been identified, officials said.

Army troops and police immediately cordoned off the area and were carrying out an initial investigation and checking security cameras for any indication of who may have been responsible for the attack. Security checkpoints were set up around the city.

The deadly blast set off a security alarm beyond Marawi city as the Christmas season ushered in a period of travel, shopping sprees and traffic jams across the country.

The Philippine coast guard said it has ordered all its personnel to intensify intelligence-gathering, stricter inspections of passenger ferries and the deployment of bomb-sniffing dogs and sea marshals following the suspected bomb attack.

“Amid this barbaric act, best public service must prevail,” coast guard chief Admiral Ronnie Gavan said in a statement.

Philippines Deadly Explosion
Filipino troopers man a checkpoint near the site where a bomb exploded in Marawi, southern Philippines (Froilan Gallardo/AP)

Presidential adviser Carlito Galvez, a former military chief of staff who now oversees government efforts to end Muslim and communist insurgencies, strongly condemned what he called a bombing incident.

“This horrendous attack, which happened during a Mass … shows the ruthless methods that these lawless elements will utilise to sow fear, anger and animosity among our people,” he said in a statement. “We will not allow this to happen.”

There was no clear indication of who was responsible for the explosion but police said they would check the possible involvement of Muslim militants, who still have a presence in the region despite years of military and police offensives.

Regional police director Brigadier General Allan Nobleza said investigators are assessing if the explosion was caused by a home-made bomb or a grenade, and if the attack was connected to the killing of 11 suspected Islamic militants in a military offensive backed by air strikes and artillery fires on Friday near Datu Hoffer town in southern Maguindanao province.

Philippines Deadly Explosion
Dozens of people were taken to hospital after the blast (Froilan Gallardo/AP)

Brig Gen Nobleza said the killed militants belonged to Dawlah Islamiyah, an armed group that has aligned itself with the Islamic State group and still has a presence in Lanao del Sur province, where Marawi city is located.

The mosque-studded city came under attack from Islamic militants aligned with the Islamic State group in 2017, leaving more than 1,100 dead, mostly militants, before the five-month siege was quelled by Filipino forces backed by air strikes and surveillance planes deployed by the United States and Australia.

The southern Philippines is the homeland of minority Muslims in the predominantly Roman Catholic nation and the scene of decades-old separatist rebellion.

The largest armed insurgent group, the Moro Islamic Liberation Front, signed a 2014 peace deal with the government, considerably easing decades of fighting. But a number of smaller armed groups rejected the peace pact and press on with bombings and other attacks while evading government offensives.