A powerful explosion tore through a chemical laboratory in central Jakarta today, ripping the roof of a nearby school building and injuring at least 22 people, one critically.

The accidental blast was triggered by chemicals stored in the laboratory, on the grounds of Indonesia's Food and Drug Agency, Jakarta police chief Maj Firman Gani said at the scene.

The explosion panicked nearby residents.

The city in recent years has seen a series of bomb attacks, mostly blamed on the al-Qa'ida-linked Jemaah Islamiyah terror group. Authorities have warned that the group is plotting more attacks.

Police spokesman Brig. Gen. Anton Bachrul Alam initially said one person had died in the blast, but later said he had been misinformed and that the victim was in a critical condition in hospital.

At least 21 others were injured, most of them lightly, he said.

The explosion wrecked the laboratory, on the first floor of three-storey building.

It also ripped the roof of an adjoining school building, where pupils were taking a morning test. Most of the victims were students.