Rioting and a fire in the cells of a Venezuelan police station in the central city of Valencia killed 68 people yesterday, according to the government and witnesses.

Families hoping for news outside the police station were dispersed with tear gas and authorities did not give information until late into the evening.

"The State Prosecutor's Office guarantees to deepen investigations to immediately clarify what happened in these painful events that have left dozens of Venezuelan families in mourning," said Chief Prosecutor Tarek William Saab on Twitter.

Venezuelan prisons are notoriously overcrowded and filled with weapons and drugs. Riots leaving dozens dead are not uncommon.

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State official Jesus Santander said the state of Carabobo was in mourning after the incident in Valencia.

"Forensic doctors are determining the number of fatalities," Mr Santander said.

A policeman was shot in the leg and was in a stable condition and firefighters had extinguished the flames, he said.

Many Venezuelan prisons are lawless and have been for decades. Prisoners often openly wield machine guns and grenades, use drugs and leave guards powerless.

"There are people who are inside those dungeons (...) and the authorities do not know they exist because they do not dare to enter," said Humberto Prado, a local prisons rights activist.

A video posted on Twitter showed dozens of people demanding information in front of police guarding the site.

"I am a desperate mother. My son has been here a week. They have not given any information," a woman told local media.

Venezuela's prisons suffer from dire overcrowding and a shortage of basic supplies, struggling under the deepening economic crisis that is gripping the once wealthy oil-producing country.

Because of the lack of space in prisons, convicts are often sent to police holding cells like the ones in Carabobo, meant to be used as temporary pens for suspects facing charges or court hearings, where detainees are supposed to spend a maximum of 48 hours.

Rights' groups estimate that the temporary detention centres have been overfilled by five times their capacity.

Two weeks ago, 58 detainees escaped holding cells on Margarita Island, a favoured tourist spot, when a hole opened up in their facility's wall. They were all soon recaptured.

In August 2017, a riot left 37 dead and 14 wounded in police cells in the southern state of Amazonas, while an April 2017 clash between rival gangs left 12 dead and 11 injured in the Puente Ayala prison in the eastern city of Barcelona.

A month before that, the remains of 14 people were found in a mass grave in the General Penitentiary of Venezuela, in San Juan de Los Morros in the country's mid-region.