Media reports that the explosion caused panic among dozens of students and teachers in a gymnasium where Mass was taking place, at Mindanao State University in Marawi, capital of Lanao del Sur province. At least four people were killed, and nearly 50 others were brought to two hospitals for treatment, authorities said. The explosion took place at around 7 a.m. local time.

Later that day, Islamic State militants claimed responsibility for the deadly blast, according to Reuters and The New York Times.

After praying the Angelus, Pope Francis assured the attack's victims of his prayers. He added that "I am close to the families, to the people of Mindanao who have already suffered so much."

In a telegram, addressed to Bishop Edwin de la Peña of Marawi, the pope also said he was "deeply saddened to be informed of the injuries and loss of life caused by the bombing," according to Vatican News.

Nearly 80% of the population in the Philippines -- which according to U.S. government estimates has 114.6 million people -- is Catholic. According to a census conducted by the Philippine Statistics Authority, about 6% of the population identifies as Muslim.

In 2017, Marawi saw a five-month battle between government forces and Islamic militants aligned with the Islamic State, which left over 1,000 dead, including some civilians, and displaced around 100,000 people. International forces helped the army to regain control of the besieged city.

Prior to Islamic State militants claiming responsibility, authorities had called the explosion a "terror attack," and regional military commander Maj. Gen. Gabriel Viray III said they were trying to identify those responsible, according to Reuters. The motive was yet to be determined, but police were investigating the possibility of Muslim militants' involvement.

Philippines President Ferdinand Marcos Jr. condemned the attack on the social media platform X Dec. 3, offering condolences to the victims' loved ones, urging calm and promising that the perpetrators will be brought to justice.

"I condemn in the strongest possible terms the senseless and most heinous acts perpetrated by foreign terrorists," he said. "Extremists who wield violence against the innocent will always be regarded as enemies to our society."

He added that the armed forces and police have been instructed "to ensure the protection and safety of civilians and the security of affected and vulnerable communities."

Lanao del Sur Gov. Mamintal Alonto Adiong Jr. was swift to condemn the attack. "Here in my province, we uphold basic human rights, and that includes the right to religion," said Adiong, who has visited with victims at a nearby hospital, according to ABS-CBN media network.

"Terroristic attacks on educational institutions must also be condemned because these are places that promote the culture of peace and mold our youth to be the future shapers of this country," Adiong said.

Mindanao State University said it was "deeply saddened" and "appalled" by the act of violence, saying that "violence has no place in a civilized society, and it is particularly abhorrent in an institution of higher learning like MSU."

"We unequivocally condemn in the strongest possible terms this senseless and horrific act and extend our heartfelt condolences to the victims and their families," it said in a statement posted on Facebook. "Our immediate priority is to ensure the safety and well-being of all constituents, particularly our Christian community."

The university said its classes were suspended until further notice and that additional security staff would be present on its campus.

In an update, the university said that after working with authorities, it was determined that there was no need to evacuate the campus or to put it in lockdown, but it urged caution and asked students to "remain in their dormitories, boarding houses, and homes inside the campus until further notice."

According to ABS-CBN, Cardinal Orlando B. Quevedo, a member of the Council of Leaders in the Bangsamoro Autonomous Region in Muslim Mindanao, also strongly decried the attack and sent his condolences to the victims' families.

"The massacre at the Mindanao State University Gym in Marawi City during Sunday Mass this morning is a crime that literally cries out to heaven," he said. "Perpetrated on the 1st Sunday of Advent, a season of hope, and at the beginning of the Mindanao Week of Peace, the massacre is the most terrible and most damnable terroristic crime against innocent worshippers on a Christian holy day."

At a press conference with Defense Secretary Gilberto Teodoro, Gen. Romeo Brawner, chief of staff of the Armed Forces of the Philippines, said that the military had killed 11 Islamic militants Dec. 1. They were members of the Dawlah Islamiyah, a group that has aligned with the Islamic State group and that, according to AP, still has a presence in Lanao del Sur province. There were also two other operations in the following days in Mindanao that targeted Islamic militants.

Brawner said military leaders were working to determine whether the university attack was in retaliation to the gains of the military against militant groups.

According to The Associated Press, the Dec. 3 explosion set off alarms beyond the Lanao del Sur province area, as the Advent and Christmas season "ushered in a period of travel, shopping sprees and traffic jams" across the predominantly Catholic country.

The Philippine coast guard "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," AP said.

In the capital city of Manila, Filipino Muslims gathered to pray in solidarity with the victims of the bombing attack, according to photos from news agencies.

The Dec. 3 telegram with the pope's message was signed by Vatican Secretary of State Cardinal Pietro Parolin, who was representing Pope Francis at COP28 in Dubai. It assured the people of the Holy Father's spiritual closeness amid this tragedy, commended the souls of those who died to God's mercy, and prayed for "the divine gifts of healing and consolation upon the injured and bereaved."

The pope concluded his message, praying that "Christ the Prince of Peace Christ the Prince of Peace will grant to all the strength to turn from violence and overcome every evil with good."