A fire tore through an overcrowded Indonesian prison in the early hours of Wednesday when most inmates were asleep, killing 41 people and injuring dozens of others, an official said.

"Forty-one inmates died, eight were badly injured and 72 others sustained minor injuries," Jakarta police chief Fadil Imran told a press conference.

Firefighters extinguished the blaze, which was mostly contained within one block that housed prisoners jailed on drug charges, at around 3 a.m. and evacuated the victims. 

Authorities were still investigating the cause of the incident at Tangerang Penitentiary, just outside the capital Jakarta, but suspected an electrical fault might have been to blame.

(With input from AFP)