The Bureau of Corrections (BuCor) said a "free-for-all scuffle" erupted at 8.39 a.m., inside the maximum-security compound of the New Bilibid Prison, adding that an investigation into the incident is underway, Xinhua news agency.
Justice Undersecretary Emmeline Villar confirmed the casualties, adding, "but we were told this might not be final yet", reports Xinhua news agency Monday's riot is the second deadly brawl in the prison within a month.
The last one, on October 9, left nine dead and seven others injured.
Designed to accommodate 6,435 inmates, the New Bilibid Prison currently houses 28,885 inmates, or a congestion rate of 349 percent, showed official data.
Overcrowding is a common problem in the prisons in the country.