Indonesian police are investigating whether a high school student was radicalised in the lead-up to a Jakarta explosion that injured almost 100 people.

Almost 30 people, mostly school students, are being treated in hospital after at least two explosions at a North Jakarta mosque during prayers on Friday.

A 17-year-old student at the school next to the mosque, who is suspected of carrying out the attack, remains in hospital.

Police said they found what they described as a "toy weapon" at the scene, which had some inscriptions on it.

Photos shared online appear to show the toy gun, with the words "Brenton Tarrant. Welcome to Hell" written on it.

Brenton Tarrant is an Australian far-right extremist who killed 51 worshippers at a mosque in Christchurch, New Zealand in 2019.

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