Police said 55 people were in hospitals with a range of minor to serious injuries, including burns, after the blasts at the mosque inside the school.

"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."

House of Representatives' Deputy Speaker Sufmi Dasco Ahmad, speaking to media after visiting a hospital, said the young male suspect was undergoing surgery, without giving more details or possible motive.

National Police Chief Gen. Listyo Sigit Prabowo said the suspect was a student at the school, and an investigation was underway into his background and motive.

"We have identified the suspected perpetrator, and we are currently investigating the perpetrator's identity, his environment, including his home and other things," he said.

Police found a "toy weapon" at the scene with some inscriptions on it, the Presidential Palace said in a statement, without elaborating.

Indonesia does have a history of attacks on churches and Western targets - but not mosques or schools. Islamist militancy has largely been suppressed in recent years.

Police cordoned off the iron-gated compound as a crime scene, with black-clad officers carrying assault rifles while emergency vehicles and armored vehicles lined up in the street.

The complex is in a crowded area on largely navy-owned land, home to many military personnel and retired officers.

At the scene, a line of shoes stood outside the green-painted mosque, while forensics combed through evidence. A damaged alms box and fan lay on the ground but there appeared to be no major structural damage to the exterior.

"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," said Immanuel Tarigan, a resident who lives in the area.

News agency Antara quoted the deputy chief security minister Lodewijk Freidrich as saying there were two explosions.