The World Organization against Torture, known by its French acronym OMCT, told AFP that around 1,000 people were arrested and detained in the capital N'Djamena during and after the October 20 protests.
Another 1,100 people had been detained in the Moussoro prison, around 300 kilometres from the capital, and in the high-security Koro Toro prison located in the middle of the desert, it said.
Two Chadian rights activists in Geneva this week to attend a review of their country's record by the United Nations Committee Against Torture voiced alarm at developments and called for the international community to do more to stop the violence.
During the curfews put in place in a number of cities, "they go to people's homes and arrest them," Mahamat Boukar Adoum, interim head of the Chadian Human Rights League (LTDH), told AFP.
"This is still going on. People are taken duing the night They are brought to Koro Toro, the 'Chadian Guantanamo'," he said, drawing comparison to the US-run Guantanamo Bay detention facility in Cuba.
"People have no contact with their family, their lawyers. There is no official list of the people detained," he said.
"It is total confusion."
Isidore Collins Ngueuleu Djeuga, who heads OMCTs African rights division, said that Chad's judicial authorities were due to go to Koro Toro this week to inform the detainees of the charges against them.
The Geneva-based NGO called for all of those detained to be released, or at the very least be granted a fair trial, and demanded security guarantees for their lawyers as they cross the desert to the prison.
The authorities say 50 people died, including a dozen members of the security forces, and accused the opposition of mounting an "insurrection."
OMCT said Monday that as many as 115 may have died, and has accused the security forces of summary executions and torture.
Agnes Ildjima Lokiam, head of the Chadian Association for the Promotion and Defense of Human Rights (ATPDH), told AFP the heads of opposition parties and organizations "have gone into hiding" because they fear for their lives.
"We are demanding an independent, impartial and international investigation."
Opposition groups called the protests to mark the date when the ruling military had initially promised to hand over power -- a timeline recently extended for another two years by General Mahamat Idriss Deby Itno.
The 38-year-old succeeded his father, Idriss Deby Itno, who ruled for 30 years before being killed during an operation against rebels in April 2021.