JAKARTA: Dozens of people were injured and hospitalised after an explosion shook a mosque at a school complex in Indonesia's capital Jakarta on Friday (Nov 7).

The incident during Friday prayers could have been an attack, officials indicated, with a 17-year-old identified as the"suspected perpetrator".

The country's deputy house speaker Sufmi Dasco Ahmad, speaking to the media after visiting a hospital, said the suspect was currently undergoing surgery.

Speaking at a press conference on Friday evening, national police chief Listyo Sigit Prabowo said the suspect is a student at the school and an investigation is underway into his background and possible motive.

Listyo said there were so far no fatalities from the explosions in Jakarta.

“Regarding the suspect, we already got the person. And authorities are trying to find out the identities of the victims," he said.

“What is certain, there are no deaths so far. Two are undergoing surgery and the rest are being treated,” he said, adding that one of the two undergoing surgery is the suspect.

Police are investigating the cause of the explosion at the site in Kelapa Gading, North Jakarta, city police chief Asep Edi Suheri told a televised press conference earlier in the day. The site is on largely navy-owned land, and is home to many military personnel and retired officers.

A total of 55 people were admitted to hospitals, he said, adding that the injuries ranged from minor to serious and included burns.

Local news agency Antara reported that the National Police's Special Detachment anti-terror unit, also known as Densus 88, was investigating whether there were any elements of terrorism in the explosion.

Witnesses told local television stations that they heard at least two loud blasts around midday, just as the sermon had started at the mosque at SMA 72, a state high school within a navy compound.

State news agency Antara also quoted the deputy chief security minister Lodewijk Freidrich as saying there were two explosions at the mosque.

Students and others ran out in panic as grey smoke filled the mosque.

"We were so surprised by the sound, it was massive. Our hearts were beating fast, we couldn't breathe, and we ran outside," said Luciana, 43, who was working at the school canteen at the time. She described multiple blasts, broken windows and panic as dozens fled the complex.

"I thought it was an electrical wiring problem, or the sound system exploded, but we didn't know exactly what it was because we ran out just as a white smoke billowed from the mosque."

Earlier, the Jakarta Fire and Rescue Agency said that the explosion originated from a speaker at the school.

Sulikha Sutiii, 73, a food stall owner who operates a business about 50m from the school, told CNA: “I heard two loud explosions like bomb sounds: ‘dor’. But I thought perhaps those were trucks which collided with one another."

She added that the authorities started to arrive at the school about 10 minutes after the explosions.

When CNA arrived at the scene at about 5.30pm local time, about 100 people – including members of the public and media - were seen gathering in front of the school, which had its gates shut.

But some soldiers, dressed in military gear, were seen leaving and entering the school which is closed to the public.

Within half an hour, dusk begun to settle over North Jakarta, and the surrounding areas around the school began to get dark.

Asep said an anti-bomb squad that was deployed at the scene found toy rifles and a toy gun near the mosque.

"Police are still investigating the scene to determine the cause of the blasts,” Suheri said, and urged against speculation that the incident was an attack before the police investigation is completed.

“Let the authorities work first,” Suheri said. “We will convey whatever the results are to the public.”

Dante, a student who was attending prayers at the mosque, told news outlet Kumparan that the explosion threw nails in the air, injuring five of his friends.

“(Two of them) were hit in the hands and feet, hit by nails," he said, initially believing that the explosion came from the mosque’s sound system.

Local resident Immanuel Tarigan added:"I was looking for our kids who go to school there. It was crowded, we saw a lot of injured victims, there were some whose faces were destroyed."

Antara reported that authorities were alerted to the explosion at about 12.10pm local time, and that two fire trucks were deployed to extinguish the fire.

News channels KompasTV and MetroTV showed footage of a police line around the school with ambulances standing by. Images of the mosque showed no extensive damage.

According to CNN Indonesia, Deputy Coordinating Minister for Political and Security Affairs Lodewijk Freidrich Paulus said that a photo of a purported weapon, circulating on social media, is “just a toy”.

"There was a picture of it, but it turned out to be a toy gun, not a real gun, after we checked it was a toy gun," he was quoted as saying by the media outlet, stressing that authorities are still investigating the incident.