Sixty-one people were reported killed and 120 injured in a prison riot in Venezuela after troops entered the complex to confiscate weapons.
Prisons Minister Iris Varela said the troops were sent in after two days of fighting inside the Uribana prison complex.
Outside the cordoned-off prison, a couple hundred people gathered to await news of loved ones.
"The guards tell us nothing" Noelvis Coyantes, 32, said. "I don't know if my brother is dead or alive."
Vice President Nicolás Maduro, who has stood in for ailing President Hugo Chávez for more than six weeks, called the violence tragic and said authorities have launched an investigation.
The authorities have yet to provide an official death toll and say they have begun to evacuate the prison while investigating.
Humberto Prado, an activist who leads the watchdog group Venezuelan Prisons Observatory, said inmates' relatives and media accounts put the toll at 55 killed and 88 injured.
The Venezuelan newspaper Ultimas Noticias and the television channel Globovision reported more than 50 killed, both citing Ruy Medina, the director of Central Hospital in the city.
Medina told the Associated Press on Saturday that the number of dead from the riot at Uribana prison had risen to 61.
Henrique Capriles Radonski, who lost against Chávez in Venezuela's October presidential elections, condemned the government's record on prisons.
"Our country's prisons are an example of the incapacity of this government and its leaders. They never solved the problem," Capriles said on his Twitter account. "How many more deaths do there have to be for the government to acknowledge its failure and make changes?"
Last year, the Uribana prison reportedly held gladiatorial contests, fights organized by gang leaders in which contenders battled to the bloody end for the entertainment of inmates, according to local media reports. It was one of these bouts that left two dead and 128 injured in February at the site.
About 500 people were killed in Venezuela's prisons in 2011, according to the Venezuelan Prisons Observatory.
The country's 34 prisons house nearly 50,000 people, but were built for less than a third of that, according to local rights groups. Uribana was built for 850 people but currently holds about 1,400. Machine guns, rifles and grenades are ubiquitous in Venezuela's notoriously overcrowded jails, and riots are commonplace.
"We face a truly serious prison crisis in which the state has not shown up with solutions, and this has led to chaos," said Carlos Nieto Palma, who runs Window to Freedom, an organization that monitors prisons.
"They have the types of weapons that can only be obtained by the country's armed forces," Nieto added. "No one else has these."
In August, 25 were killed in a gang fight in Yare prison, just south of Caracas. The capital's La Planta jail had faced a similar situation in April and May when authorities spent two weeks attempting to control rioting before closing down the complex.
In June 2011, more than 5,000 troops spent a month trying to quell an uprising at El Rodeo jail, just outside Caracas. Venezuela's deadliest prison incident took place in 1994, when 130 prisoners were burned or hacked to death at Sabaneta jail in Maracaibo.