Several were hospitalized with injuries and at least 15 reported sexual assaults, including rape.
Warning: very disturbing content. 

The organizers of the aid shipment issued a statement alleging that Israelis threw detainees into containers on a prison ship and beat them over the head and ribs. The statement added that "at least 12 sexual assaults have been documented on that vessel alone, including anal rape and forcible penetration by a handgun." 

Israel denies the allegations. There is a history of rape accusations against Israelis. In 2024, guards at the Sde Teiman military base and prison in Israel were accused of raping and torturing a Palestinian man. Video tape of the event was later released.  ~ JW Williams

Warning: video depicts injuries from violence

From the Times of Israel:

Activists released from Israeli custody after being detained on a flotilla trying to break Israel's naval blockade on Gaza were subjected to abuse, organizers alleged on Friday, with several hospitalized with injuries and at least 15 reporting sexual assaults, including rape.

The Israel Prison Service denied the allegations, saying all detained activists were held "in accordance with the law, with full regard for their basic rights," and received necessary and professional medical care.

Reuters noted it was unable to independently verify the activists' claims.

Germany said some of its nationals had been injured and that some accusations were "serious," without giving further details.

A legal source in Italy said prosecutors there were investigating possible crimes, including kidnapping and sexual assault.

Israeli forces arrested 430 people on board 50 ships in international waters on Tuesday to halt a flotilla of volunteers trying to break the blockade on Gaza.

The allegations of abuse will add pressure on Israeli authorities to explain the treatment of the detainees, after a video of National Security Minister Itamar Ben Gvir in a detention center mocking some of the bound, kneeling activists sparked an international outcry. Ben Gvir posted the video himself.