At least 37 people were killed and 14 law enforcement personnel wounded when armed inmates resisted authorities' attempt to impose order at a prison in the Venezuelan state of Amazonas, officials said.
"The deaths occurred during a seizure of the Amazonas Judicial Detention Center (CDJA) in which 14 officials were also wounded," the Attorney General's Office said in a statement on Wednesday.
Amazonas Governor Liborio Guarulla, an opponent of President Nicolas Maduro, described the episode as a "massacre" caused by an Interior Ministry team that tried to take the prison by force, Efe news reported.
A source with first-hand knowledge of the events said that the trouble began during a search of the CDJA by a police task force.
The 37 inmates killed represented nearly 40 per cent of the jail's current population, the source said.
Venezuela, with 166 people behind bars for every 100,000 residents, has the region's third-smallest prison population, yet its penitentiary system is among the most violent.
As part of an effort to curb prison violence, the government has imposed a militarized administration in roughly half of Venezuela's penal institutions.