"Chadian authorities need to immediately ensure that an effective, independent investigation will be conducted to determine if the security services' use of lethal force was a justified, proportionate response to any alleged violence," said Lewis Mudge, Human Rights Watch's Central Africa director.
"People should be able to peacefully protest government policy without being shot at or killed," he said in a statement issued late Wednesday.
Authorities say 50 people died, including a dozen members of the security forces, and accused the opposition of mounting an "insurrection" with the protests last Thursday marking 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.
A Geneva-based group called World Organization against Torture (OMCT) has given a provisional toll of at least 80 deaths, and accused the security forces of summary executions and torture.
The African Union (AU) and European Union (EU) have issued statements condemning the crackdown and violation of freedom of speech and assembly.
The 38-year-old Deby succeeded his father, Idriss Deby Itno, who ruled for 30 years before being killed during an operation against rebels in April 2021.