Congestion in the jail in a Manila suburb is likely to have contributed to the fight between two prison gangs that broke out on Friday, said the jail's duty officer Norberto Felicen.
"We have 3,424 inmates in a facility built to hold 800 people. Wherever there is an open space, they have to use it to sleep. That is one of the factors that caused the riot," Felicen told AFP.
The trouble began when members of the "Bahala Na" gang, detained at the Quezon City jail, brought a gang member who had suffered a seizure to the jail's clinic, Felicen said.
In their rush, they overturned a pail of water on to the sleeping members of a rival gang who awakened in anger, thinking they were being provoked.
The members of the two gangs battled each other with rocks and chairs before order was restored, but not before one inmate was killed.
The prisoner who suffered a seizure also died in the clinic, Felicen said, adding that the incident is being investigated.
Quezon City jail received international attention last year after AFP photographs showed massive overcrowding made worse by President Rodrigo Duterte's draconian anti-drug campaign.
The pictures showed detainees jammed in like sardines, taking turns sleeping on a staircase and an open-air basketball court.
Human Rights Watch criticised the conditions at the time, saying it was "straight out of Dante's 'Purgatory'", referring to the 13th century Italian writer's description of the afterlife.
Authorities had promised to build new facilities to alleviate the overcrowding but said this would take time.
Even before Duterte took office last year, the Philippine penal system was ranked as the third most congested in the world, according to the University of London's Institute for Criminal Policy Research.
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