Manila, Dec 3 : At least four persons were killed and 50 others injured in an explosion while people gathered for a Catholic mass inside a gym at the Mindanao State University in the Philippines' southern province of Lanao del Sur on Sunday, according to the military and police. Major General Gabriel Viray, the commander of the Army's 1st Infantry Division, said the victims included three females and a male, reports Xinhua news agency. The blast occurred at around 7 a.m. when students and teachers had gathered for the mass inside the gym, according to the police. Police were investigating the type of explosive device used in the blast. 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 also deployed additional security personnel to safeguard the campus, located in Marawi City, which is the capital and the largest city of Lanao del Sur on Mindanao island. President Ferdinand Marcos Jr condemned the blast as a "senseless and most heinous" act, which was "perpetrated by foreign terrorists". He appealed to the public to remain calm. "Rest assured, we will bring the perpetrators of this ruthless act to justice," he said. In 2017, local terrorists pledging support for 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 and hundreds of thousands of residents displaced. Nearly 80 per cent of the country's 113 million population are Catholic and it is not uncommon for school gymnasiums and even shopping malls to designate areas for Sunday Mass, especially in places where there are no churches. /IANS