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 network Milenio said the shootout was triggered by the arrest of Ovidio Guzman, one of the sons of "El Chapo," who have assumed control of part of the cartel since their father was extradited to the United States in 2017.
Other media reports said one of Guzman's sons had been killed.
Authorities did not immediately confirm or deny either report.
President Andres Manuel Lopez Obrador said in a brief comment to journalists that his security cabinet was holding a meeting and would give details on the situation later.
The Sinaloa state government said police officers had been wounded, but did not give a detailed toll.
It also said some inmates had escaped from the Aguaruto prison in Culiacan amid the chaos, but did not say how many.
It 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."
The battle broke out in the afternoon near the state prosecutor's office, when masked gunmen blocked traffic and opened fire, causing panicked drivers to abandon their cars in the middle of the street.
The fighting then spread to several other parts of the city.
Gunmen blocked roads and highways around the city into the evening, bringing the capital to a standstill.
Drug trafficking bastion
"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.
Guzman's extradition unleashed an initial period of instability in the group, as his sons waged war with his cartel co-founder, Ismael "El Mayo" Zambada, for control, leaving a trail of bodies in their wake.
But the situation has since stabilized into a reluctant truce.
Guzman -- whose nickname translates as "Shorty" -- is considered the most powerful drug lord since Colombia's Pablo Escobar, who was killed in a police shootout in 1993.
After being convicted in a New York court, he is now serving his sentence in the notorious ADX federal maximum security prison in the US state of Colorado, nicknamed the "Alcatraz of the Rockies."