Vice President Nicolas Maduro called the violence tragic early Saturday on television and said the authorities had launched an investigation.
He and other officials did not give a death toll from the riot at Uribana prison in the central city of Barquisimeto.
The newspaper Ultimas Noticias reported on its website that 54 were killed. The television channel Globovision reported about 50 killed. Both cited Ruy Medina, the director of Central Hospital in the city, who also said that dozens were hurt.
Penitentiary Service Minister Iris Varela said earlier on television that the riot broke out when groups of inmates attacked National Guard troops who were attempting to carry out an inspection.
Varela said the violence had affected a number of prisoners and officials, but said the authorities would hold off until control had been re-established at the prison to confirm the toll. She said the government decided to send troops to search the prison after receiving reports of clashes between groups of inmates during the past two days.