Seven inmates were killed in a prison riot in southern Mexico, when inmates resisting transfers to other jails fought police with guns and knives, authorities said late Thursday.

Four police officers and six other inmates were injured in the riot in the city of Villahermosa, the capital of the Gulf coast state of Tabasco.

State police chief Víctor Hugo Chávez said late Thursday that officers were met by gunfire early in the day when they entered the prison to transfer two dangerous inmates to a federal penitentiary.

One of the inmates with a gun held out for about three hours, protected by 20 fellow prisoners, officials said.

Chávez did not specify if police opened fire on the group, but said: “Authorities have to act to defend their own lives, too.”

Fires also broke out in the prison during the riot, and crowds of angry, desperate relatives gathered outside the prison, demanding information about family members locked up inside.

After regaining control of the facility several hours later, authorities found an assault rifle, five pistols, a hand grenade, 23 machetes, 14 knives and 23 homemade shivs.

They did not explain how the firearms got into the prison. Jails in Mexico are known for notoriously loose controls and corruption, to the extent that gangs at some penitentiaries control their own cell blocks and extort other prisoners for protection money.

Last year, an attack on a state prison in Ciudad Juarez across the border from El Paso, Texas killed 17 people, including 10 guards. Twenty-five inmates escaped in the attack, which authorities say was designed to free the leader of a local gang.

In 2016, a prison riot left 49 inmates hacked, beaten or burned to death at the Topo Chico prison in Mexico.