The human rights organisation Cristosal says that at least 261 people have died in prisons in El Salvador during President Nayib Bukele's two-and-a-half-year-old crackdown on street gangs.
Under a state of emergency originally declared in 2022 and still in effect, Bukele's government has rounded up 81,110 suspected gang members in sweeps that rights groups say are often arbitrary, based on a person's appearance or where they live.
The group said in a report that, as of April 15, 88 of the 261 deaths "may have been the result of a criminal act," though it did not specify what those acts may have been.
The report said 87 of the deaths were due to illnesses, 14 were apparently "acts of violence" and no cause could be immediately identified for the other 72.
"People have died in El Salvador's prisons and jails because of torture, a lack of food, unhealthy conditions, an inhuman lack of attention and cruel, inhuman and degrading treatment," said lawyer Zaira Navas, who authored the report.
"There is a deliberate policy of not protecting the rights of incarcerated people."
While the government is accused of committing mass human rights abuses in their crackdown, Bukele remains highly popular in El Salvador because the homicide rates sharply dipped following the detentions.
The Central American nation went from being one of the most dangerous countries in the world to having the lowest homicide rate in the region.
Bukele rode that popularity into re-election in February, despite the country's constitution prohibiting second terms for presidents.
The government has already had to release 7000 people due to lack of evidence and El Salvador's vice president in January told the Associated Press the government had "made mistakes" in their arrests.
The rights group estimates that of the people arrested in the two years of the exception regime, 35 per cent are innocent and affirms that 94 per cent of the deceased had no gang affiliation.
"The majority were working people such as informal traders, cab drivers and/or informal transport workers, farmers, fishermen, evangelical pastors and preachers, municipal employees and one trade unionist," an earlier report from Humanitarian Legal Relief stated.
Humanitarian Legal Relief also demanded El Salvador's government investigate "homicides" that have occurred in prisons and "all the forced disappearances of detainees."
El Salvador's legislators granted a request by President Nayib Bukele for the 24th consecutive one-month extension of an anti-gang emergency decree that seen tens of thousands of people detained in prison.
The vote by congress meant that by March 27, the country will have spent a full two years under the decree, which suspends some rights.
Bukele has used emergency powers to round up 78,175 suspected gang members in sweeps that rights groups say are often arbitrary, based on a person's appearance or where they live.
The government has had to release about 7000 people because of a lack of evidence.
The original 30-day state of emergency — approved on March 27, 2022, following a spate of 62 killings in one day — restricts the right to gather, to be informed of rights and have access to a lawyer. It extends to 15 days the time that someone can be held without charges.
Suspects rounded up are held in the mega-prison called the Terrorist Confinement Centre under a harsh regime.
The Terrorist Confinement Centre was built after tough new laws were announced to crack down on gang violence in the Central American country.
The justice minister vowed that those sent to the prison will "never return" to the streets.
The tough statement came as the administration of President Nayib Bukele asked for yet another extension of an anti-gang emergency measures that would take the crackdown into its 13th month.
Over the last 354 days, about 65,000 people have been arrested in the antigang campaign.
Human rights groups say that there have been many instances of prisoner abuses and that innocent people have been swept up in police raids.
The government announced the mass inmate transfer with a slickly produced video posted on social media.
It showed prisoners forced to run barefoot and handcuffed down stairways and over bare ground, clad only in regulation white shorts.
They were then forced to sit with their legs locked in closely clumped groups in cells.
Gustavo Villatoro, the government's minister for justice and peace, said the suspected gang members would never return to the streets, even though tens of thousands of those arrested are still awaiting formal charges or a trial.
Bukele, who revels in taking a contrarian stance and once described himself as "world's coolest dictator," wrote in his Twitter account that "there are now 4000 gang members in the world's most criticised prison".
Dubbed the Terrorism Confinement Centre, the prison was inaugurated in February and already holds about 2000 suspected gang members.
Armoured guards watch over prisoners.
Bukele requested the special powers to pursue the gangs last March 27, following a surge in gang violence in which 62 people were killed in a single day across the country.
It is a sprawling campus east of San Salvador, the capital, that could eventually house up to 40,000 inmates.
Street gangs like MS-13 and Barrio 18 have long killed and extorted money from residents in El Salvador.