Thunberg told a news conference in Stockholm on Tuesday that she and others were "kidnapped and tortured" by the Israeli military.
She declined to elaborate, adding when pressed that she didn't get clean water and that other detainees were deprived of critical medication.
"Personally, I don't want to share what I was subjected to because I don't want it to make headlines and 'Greta has been tortured', because that's not the story here," she said, adding that what they were subjected to paled in comparison to what people in Gaza experienced daily.
Israel's foreign ministry did not immediately respond to a Reuters request for comment but has repeatedly denied mistreating the detainees.
"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," a foreign ministry spokesperson told Reuters last week.
Thunberg was part of the Global Sumud Flotilla, a group of vessels that tried to reach Gaza to bring aid supplies and draw attention to the plight of the enclave, where most of the 2.2 million residents have been driven from their homes and the United Nations says hunger is rampant.
Thunberg was detained along with 478 people in the flotilla and expelled from Israel on Monday.
Israel, which says reports of hunger in Gaza are exaggerated, has dismissed the flotilla as a publicity stunt benefiting Palestinian militant group Hamas.
It had previously detained Thunberg at sea in a similar attempt to breach Israel's blockade of Gaza in June.
Swedish activists said on Saturday that Thunberg was shoved and forced to wear an Israeli flag
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