Cameroon’s overcrowded maximum-security prison was on Thursday swept through by a raging fire sending three inmates to the hospital with severe burns.

A local fire chief disclosed that firefighters backed up by police were able to contain the blaze and stop it from spreading beyond New Bell jail, which sits in a densely populated neighbourhood of Cameroon’s port city, Douala.

“The operation was made more difficult by the layout and the large number of inmates,” fire brigade chief Kadrey Abdiel said.

Human rights groups say Cameroon’s prisons suffer from chronic overcrowding, poor sanitation conditions and disease, and cases of violence and torture are frequent. New Bell was built in the 1930s for 800 inmates, but held around 2,500 as of 2011, according to Amnesty International.

Three severely burned prisoners were evacuated, while two firefighters were injured, Abdiel said.

Crowds of onlookers had gathered outside the prison, where a large plume of dark smoke billowed from the roof.

A few inmates had tried to climb over the prison’s walls, but police around the perimeter stopped them from escaping, regional Governor Samuel Ivaha Diboua told journalists.

It was not immediately clear what started the fire.