Sixteen inmates have been killed and five wounded after a fight broke out in a Mexican prison over a football game.
The fight broke out at about 2.30pm local time on Tuesday in the Regional Center for Social Reintegration in the town of Cieneguillas , the state government said.
According to local media, the fight was triggered by a football game.
The state's public security ministry did not comment on the cause of the unrest but said the situation was now under control. A statement indicated the melee last about two and a half hours.
The ministry is working to release the names of the inmates who were killed. Of the victims, 15 died in the prison and one in hospital.
After the fight, one inmate was arrested with a firearm, while three more guns and knives were found in the prison.
Ismael Camberos Hernandez, Zacatecas state security secretary, said the prison had been searched on Saturday and Sunday and no guns had been found.
He said some victims had died of gunshots wounds, but others were stabbed or beaten.
No guards or police were wounded or killed.
An investigation has been launched to find out who was responsible for the fight and how the weapons got inside the prison.
Without mentioning the fight specifically, Mexican security Minister Alfonso Durazo said: "We will continue promoting the reorganisation of the prisons to fight the crime that is organised from there."
Security will be boosted at prisons in the Zacatecas state.
The fight is another blow to Mexican President Lopez Obrador, who took power in December 2018 and promised to reduce record levels of violence.
The country was on track to surpass the previous year's total homicides in 2019 according to latest data.
In 2017, 28 inmates were killed when a fight broke out in a prison in Acapulco.