Gang rivalry in the prison has led to the deaths of more than 2,000 over the past year
At least 20 people have been killed during a riot at a prison in the Mexican border city of Ciudad Juarez, say police sources.
Officials said many other people were injured as prisoners fought with knives and set fire to cell blocks.
Rival drugs gangs have been battling for control of Ciudad Juarez, where more than 2,000 people have been killed over the past year.
The government has already deployed more than 1,500 troops in the city.
Chihuahua state police spokesman Carlos Gonzalez told the Associated Press news agency that a fight broke out between three different gangs in the early hours of the morning.
An unnamed police official told AFP that 20 people had been stabbed or beaten to death.
Television footage showed brown smoke coming from the windows of the prison buildings as helicopters hovered overhead.
Several people could also be seen on the roof of the facility.
Soldiers and police wearing riot gear later regained control of the prison, located in the desert outside the city.
Ciudad Juarez is a key entry point for drug smuggling into the US and has become a centre of gang warfare.
Mexico hopes to deploy 7,000 soldiers to tackle violence in the city
Chief of police Roberto Orduna stepped down last week amid threats against his officers.
Criminal gangs had threatened to kill at least one police officer every two days until he resigned.
Earlier, 1,500 soldiers were moved into the city and Mexican officials say they intend to have 7,000 military personnel and police in position in the city by the end of the week.
Attorney General Eduardo Medina Mora said the situation in the city was deeply worrying and the troops were there in support of the local authorities.
The US has said it is prepared to help Mexico in its battle against drugs cartels.
US Defense Secretary Robert Gates said such aid could take the form of military hardware, training and intelligence support to tackle what he said was "a serious problem".