Some 28 inmates have been killed and three wounded after rival gangs fought inside a prison in the Mexican resort of Acapulco.
Officials initially gave a death toll of five but later raised the figure after more bodies were found scattered throughout the state prison.
Guerrero state security spokesman Roberto Alvarez Heredia said the fighting broke out at around 4am. He said security forces regained control and slowly began discovering the bodies.
State police said the prison population is more than 2,000.
Guerrero has been one of Mexico’s most violent states and the once-glamorous resort of Acapulco has struggled with a high number of killings.
"The incident began because of the constant dispute between rival groups inside the prison," Mr Alvarez said.
A brawl between rival drug gangs at an overcrowded penitentiary in northern Mexico in February resulted in 49 inmates being killed, Mexico’s deadliest prison melee in years. That clash was between inmates aligned with the Zetas and Gulf drug cartels.
In June, Mexico’s governmental rights agency issued a report that said many of the country’s prisons are unsafe, overcrowded and understaffed. The report warned of insufficient staff, unsafe procedures and poor medical care at many facilities.
The National Human Rights Commission said it surveyed conditions at over 190 state, local and federal jails. It found that inmates often controlled some areas of prisons or had contraband inside.