File picture: Timothy A. Clary/AFP

Caracas - Sixty-eight people have died in a prison riot in northern Venezuela, Attorney General Tarek William Saab said late Wednesday.

Initial reports had said that five people had died when inmates at a detention centre at the regional police headquarters in Carabobo state took a guard hostage and set fire to mattresses.

However, Saab said in a series of tweets that the dead comprised 66 men and two women, who were visiting the prison overnight.

An investigation would be launched immediately to clarify what had happened, he added, saying that four prosecutors had already been assigned to the case.

"We guarantee that we will carry out a thorough investigation to immediately clarify these painful events, which have [meant] mourning for dozens of Venezuelan families," Saab wrote.

The cells at the police headquarters are used as a temporary jail, from where detainees are taken to court to be sentenced.

Family members desperate for information about their loved ones gathered outside the police station. Television footage showed officers in combat gear pushing them back.

"There was no information, nothing!" one woman, whose son was inside the prison, said in a report shown on South American broadcaster NTN24.

The non-governmental organization Una Ventana a la Libertad (A Window to Freedom), which campaigns for prison reform in Venezuela, blamed the Ministry of Corrections and the minister responsible, Iris Valera, for the deaths.

The country's regional police were in a state of chaos due to the ministry's negligence, spokesman Carlos Nieto Palma said, and prison conditions were poor.

"This is serious and alarming," he said. "What happened today is a sign [of poor prison conditions] ... and it is experienced throughout Venezuela."

He said that when the ministry was created seven years ago, Varela had promised to build new pre-trial detention centres in each state but that so far not one had been completed.