Human Rights Watch on Monday accused a Syrian group led by former Al-Qaeda fighters of using "torture" against activists opposing their rule.
Hayat Tahrir al-Sham earlier this month cemented its control over the northwestern region of Idlib at the expense of smaller, Turkey-backed outfits.
The New York-based rights group said it documented 11 cases in which HTS "detained Idlib residents, apparently because of their peaceful work documenting abuses or protesting the group's rule".
"Six of those detained were apparently tortured," including a 16-year-old boy, it said in a statement.
"One man described being hung from a pole upside down for hours during interrogation," it said.
Another said he was locked up in something resembling a coffin for three hours, while a third said he was pushed through a tyre and beaten incessantly in a method infamously used by regime security services.
"The maximum you can do is move your shoulders a bit. And scream for help. But on several occasions, they stuffed things in my mouth so I can't scream," the detainee told HRW.
"I used to lose my consciousness a lot."
Seven of those interviewed by HRW said they were media activists or journalists who had participated in or covered protests against HTS, or were working with foreign media.
Four were made to sign a document pledging they would no longer film or report on the jihadist-led alliance.
The rights group called for the immediate release of all unlawfully held prisoners, including four still being held or missing among the 11 cases documented.
"There is no legitimate excuse for rounding up opponents and arbitrarily detaining and torturing them," said HRW's Lama Fakih.
"HTS's crackdown on perceived opposition to their rule mirrors some of the same oppressive tactics used by the Syrian government."
Syria's war has killed at least 360,000 people and displaced millions since it started in 2011 with the brutal repression on anti-government protests.