Amnesty International on Tuesday called on Lebanon to investigate the death in custody of a man detained after clashes between Sunni radicals and the army last month.
The London-based rights organisation said in a statement that 35-year-old Nader al-Bayoumi died in jail apparently after suffering severe trauma.
"Amnesty International has seen images of Nader al-Bayoumi's body, which bore signs of horrific abuse," the group said.
"A forensic pathologist who reviewed the images concluded that the bruising on the body was consistent with assault and suggested internal haemorrhage was a possible cause of death."
Bayoumi was among those arrested after clashes between the military and supporters of Salafi Sunni cleric Ahmad al-Assir in the southern town of Sidon last month.
The fighting left at least 18 soldiers dead as well as at least 11 of Assir's supporters.
Signup to our newsletter and follow us on Facebook and Twitter!
Amnesty's regional programme director Philip Luther said the decision of Lebanese authorities to hand Bayoumi's body over without explanation was "unacceptable."
"An immediate, independent and transparent investigation into his case is crucial."
The group said it had collected testimony from at least three other detainees who described being tortured.
"One, a 15-year-old boy, said he suffered electrocution and was beaten with a belt and a stick before being forced into signing a false confession."
Another described having cigarettes stubbed out of his back and an officer stamping on his face.
Last month, Human Rights Watch also accused Lebanese police of torturing detainees, particularly vulnerable inmates including drug users, homosexuals and sex workers.
The New York-based rights group said it had collected testimony from more than 50 people arrested in the past five years who described physical and sexual abuse as well as being denied access to medical care and lawyers.