Swedish activist Greta Thunberg claimed she was "hit and kicked" during her time in an Israeli prison.
The 22-year-old has also accused members of the Israeli military of writing "whore Greta" on her suitcase alongside a sketch of the Star of David.
Ms Thunberg was among 437 activists, parliamentarians and lawyers detained after Israeli forces intercepted their humanitarian aid convoy earlier this month.
The flotilla had attempted to breach the 16-year maritime blockade of Gaza.
Ms Thunberg said she was taken into a "dystopian" area enclosed with an iron fence where others had been handcuffed.
"I saw maybe 50 people lined up on their knees, handcuffed, with their foreheads to the ground," she told local media.
"They pulled me to the opposite side from where the others were sitting, and I had the flag around me the whole time. They hit and kicked me."
She said the guards lacked "empathy and compassion", allegedly detainees were "screaming" as they repeatedly requested water.
Greta claims she was 'hit and punched' during her time in Israeli custody
|ISRAEL FOREIGN MINISTRY
"When people fainted, we knocked on the cages and asked for a doctor," she added.
"Then guards came and said, 'We are going to gas you.' It was standard for them to say that."
Ms Thunberg earlier claimed she was detained in a cell infested with bedbugs, with too little food and water.
It was also alleged that she was forced to kiss the Israeli flag during her detention.
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Greta Thunberg's flotilla was intercepted by Israel earlier this month
|ISRAEL FOREIGN MINISTRY/REUTERS
Turkish activist Ersin Çelik, who took part in the Sumud flotilla, told local news: "They dragged little Greta [Thunberg] by her hair before our eyes, beat her, and forced her to kiss the Israeli flag.
"They did everything imaginable to her, as a warning to others."
Israel's foreign ministry said in an earlier statement: "All detainees ... were given access to water, food, and restrooms; they were not denied access to legal counsel, and all their legal rights were fully upheld."
It also described reports that detainees had been mistreated as "complete lies".
"They did everything imaginable to her, as a warning to others."
Ms Thunberg's flotilla, made up of around 50 boats, set sail from Barcelona at the end of August.
The Gaza-bound boats were attempting to deliver medicine and food to the Palestinian enclave.
The flotilla set sail at the end of August
| GETTYThey were intercepted by Israeli naval vessels on October 1, around 70 nautical miles away from their destination.
A spokesman for the Israeli Foreign Ministry said in a statement at the time: "Already several vessels of the Hamas-Sumud flotilla have been safely stopped and their passengers are being transferred to an Israeli port.
"Greta and her friends are safe and healthy."