Ecuadorian government has blamed disputes between drug trafficking gangs over control of prisons for the violence.
At least 68 prisoners have been killed and a dozen injured in overnight violence at Ecuador's Penitenciaria del Litoral prison.
The penitentiary, located in the southern city of Guayaquil, is the same prison where 119 inmates were killed in late September in the country's worst-ever incident of prison violence.
The latest disturbance was set off by a power vacuum following a gang leader's release, governor of Guayas province Pablo Arosemena said in a press conference on Saturday.
"The context of this situation is that there was no leader of the gang that has this cell block because a few days ago that prisoner was released ... for completing 60 percent of his sentence," Arosemena said.
"Other cell blocks with other groups wanted to subdue them, get inside and have a total massacre."
Videos on social media purportedly posted by detainees overnight showed them begging for help to stop the violence as shots and explosions sounded in the background.
The origin of the videos could not be verified.
Growing violence in prisons
There has been a wave of disturbances in the South American country's prisons, which house some 39,000 detainees.
Incidents in February and July at various prisons left 79 and 22 people dead respectively.
Some of those killed in September at the penitentiary were decapitated or burned, the attorney general's office has said, and dozens were injured.
A total of 11 people were found hung in Penitenciaria del Litoral in October, which authorities have said may have been suicides.
President Guillermo Lasso in September declared a 60-day state of emergency in the prison system, freeing up funding and allowing heightened controls with military assistance.
Police operations since the state of emergency was declared, especially at Penitenciaria del Litoral, have turned up guns, grenades, knives, munitions, cell phones and drugs.