MANILA, Philippines — A bomb exploded during Christmas Day Mass at a chapel inside a police camp in the volatile southern Philippines, wounding a priest and 10 churchgoers, officials said Saturday.

The improvised explosive was hidden in a ventilation window close to the ceiling of the chapel, which is located inside a police camp in Jolo town on Jolo Island, a stronghold of al-Qaida-linked militants, Sulu provincial police said.

Six of the wounded were brought to a nearby government hospital, and four others were sent to a Philippine marines hospital, police said. One did not need hospital treatment. None of the injuries was life threatening and everyone was later sent home, except for a woman who was under observation, according to police.

The Rev. Romeo Villanueva, 72, said a newly ordained priest, the Rev. Ricky Bacoldol, who was assisting him, was thrown off his feet by the impact of the blast and suffered a slight leg injury.

"I was reading the Gospel. I was not yet finished when there was a loud explosion," Villanueva told The Associated Press by telephone.

He said the roof over the front of the church collapsed and wooden beams and other debris flew in all directions. The organist was spared from harm when a portion of the ceiling shielded him from the blast, Villanueva said.

There were about 50 people inside the church but many more were coming in at the time, he said.

According to investigators, the explosive device appeared to have been placed inside a black plastic bag and may have been detonated through a cell phone, parts of which were recovered at the scene.

The Philippines is predominantly Catholic, but Christians are a minority on Jolo and nearby island provinces.

President Benigno Aquino III’s spokesman, Edwin Lacierda, condemned Saturday’s bombing, saying it "violates the basic tenets of respect and peace of all who hold their faith dear." He said there could be no religious or political justification for the attack.

It wasn’t immediately clear who was responsible. Similar bomb attacks in the majority Muslim island province have been blamed on Abu Sayyaf guerrillas, who have gained notoriety for high-profile kidnappings and beheadings.

The military estimates that battle setbacks, arrests and surrenders have reduced the group’s strength to more than 300 from more than 1,000 guerrillas during its heyday in 2000.

The Abu Sayyaf is on a U.S. list of terrorist organizations and is suspected of having received funds and training from al-Qaida.