Yaya Dillo, who died during an army assault on his Socialist Party without Borders (PSF) headquarters last February, had been the strongest rival to his cousin and coup leader General Mahamat Idriss Deby Itno.
Deby won the contested election and was sworn in last week after three years of military rule.
The army denied accusations of putting a bullet in Dillo's head from point blank range and alleged he had led an attack on the intelligence services to try to free a detained party official.
Deby had been proclaimed transitional president in April 2021 by a junta of generals after his father, president Idriss Deby Itno, was shot dead by rebels after 30 years in power.
Deby officially won 61 percent of the May 6 vote that international NGOs said was neither credible nor free and boycotted by an opposition ravaged by a junta crackdown.
"Chadian authorities must ensure the fair trial rights of 26 people detained incommunicado for more than three months," in a desert prison, Amnesty said, noting none of them, had been brought before a judge.
They are being detained in the maximum-security Koro Toro prison, 600 kilometers (400 miles)from the capital N'Djamena.
"All those detained must be immediately released unless they are promptly charged with an internationally recognizable offence and tried in proceedings that adhere to international fair trial standards," said Samira Daoud, Amnesty's regional director for West and Central Africa
Incommunicado detention cases were "part of a pattern of repression against opposition supporters in Chad," that has seen hundreds arrested, the rights group said.
Amnesty renewed international calls for an independent inquiry into Dillo's death, saying there was no public information about any investigation.