At least ten prisoners were killed in a prison revolt in the western provincial capital of Maracaibo, media reports said on Sunday.
Most of the victims died when a grenade exploded amidst a clash between rival gangs in the prison on Saturday, officials said.
Sixteen prisoners were injured and taken to hospital in the clashes, which included gunfire and arson.
Police and prison guards didn't get control of the situation until Sunday. Thousands of relatives of the inmates gathered before the prison to protest conditions inside and demand information about the identity of the victims.
Venezuela has about 26,000 prisoners in 30 jails. Human rights groups criticise conditions inside. In the past year, rights activists said 498 prisoners died violent deaths behind bars.
Across Latin America, prison revolts, gang clashes and attacks on the guards as well as abuse of inmates by poorly-paid prison guards are common.