Conditions are very poor in Brazilian prisons

They were among more than 80 inmates fleeing Sao Paulo state penitentiary through a tunnel linked to the sewers.

The escape bid came to light when local residents alerted jail authorities, who managed to recapture more than 40 prisoners.

Rescue workers said the tunnel had ventilation and a cement lining, and was dug from outside the prison.

In addition to the six confirmed dead, another seven prisoners - who have not yet been found - are thought to have died of suffocation.

Buried alive

The mass escape on Sunday was timed to coincide with visitors' day at the prison, which is in the north of the city.

The tunnel ran from a disused office in the jail to an empty house outside the prison walls, incorporating existing sewer tunnels.

During the escape, part of the tunnel caved in, burying the prisoners alive.

Officials said some prisoners were dragged from the network of drains, but others were believed to have died in the sewers after losing their way.