An NGO quoted local residents as saying explosions and then gunfire were heard starting late Monday, with the violence continuing overnight.
Gang fighting at a jail in Venezuela left at least 10 inmates dead, an NGO reported Tuesday.
At least four grenades were detonated in clashes that began Monday evening in the northwestern state of Zulia, said the NGO, a human rights group called Una Ventana a la Libertad.
Venezuelan authorities have not said anything about a toll from the rioting, which the NGO said broke out at a jail which serves as a holding pen at a regional police headquarters.
Prisoners set fire to one wing of the facility, it said.
It quoted local residents as saying explosions and then gunfire were heard starting late Monday, with the violence continuing overnight.
Zulia governor Omar Prieto said Monday that he is planning to close the detention centre.
On December 23, an uprising at the jail left six people dead, the NGO said. One of those was shot by security forces, it added.
It said the jail holds about 1,000 inmates.
Venezuelan prisons suffer from severe overcrowding despite a 2011 plan to upgrade them.
Holding pens like the one in Zulia are full of detainees, who are not supposed to spend more than 48 hours there.
Nationwide, these facilities hold more than five times their capacity. Detainees suffer from violence, malnutrition and diseases such as tuberculosis, La Ventana a la Libertad said.
It said that in just the first half of 2019, 149 prisoners died in police holding pens, due to a variety of causes.
On May 24, a total of 29 inmates died during an attempted breakout from another such jail in the town of Acarigua in the western state of Portuguesa.
Clashes between prisoners and police left 19 officers injured in that incident.