Heavily armed gunmen in four-by-four trucks fought an intense battle against Mexican security forces Thursday in the city of Culiacan, capital of jailed kingpin Joaquin "El Chapo" Guzman's home state of Sinaloa.
Sustained, heavy gunfire ground the western city of 750,000 people to a halt, in a battle that left blazing vehicles strewn across the street and sent terrified residents running for shelter, said AFP journalists at the scene.
Images carried on Mexican television showed army and police forces under assault by civilians armed with heavy machine guns.
Mexican TV networks said the shootout was triggered by the arrest of one of Guzman's sons, who have assumed control of part of the cartel since their father was extradited in 2017.
Other media reports said one of Guzman's sons had been killed.
Authorities did not immediately confirm or deny either report.
The Sinaloa state government said in a statement that it was "working to restore calm and order in the face of the high-impact incidents that have occurred in recent hours in various points around Culiacan."
It called on residents to "remain calm, stay off the streets and be very attentive to official advisories on the evolving situation."
"El Chapo," 62, was sentenced to life in prison in July for trafficking hundreds of tons of cocaine, heroin, methamphetamine and marijuana into the United States over the course of a quarter-century.
However, his Sinaloa cartel remains one of the most powerful in Mexico.