Activists released from Israeli custody after being detained on a flotilla trying to take aid to Gaza were subjected to abuse, organisers have alleged, with several hospitalised with injuries and at least 15 reporting sexual assaults, including rape.

Israel’s prison service denied the allegations, and Reuters was not able to verify them independently.

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.

An Israeli prison service spokesperson said in a statement: “The allegations raised are false and entirely without factual basis.

“All prisoners and detainees are held in accordance with the law, with full regard for their basic rights and under the supervision of professional and trained prison staff,” they said.

“Medical care is provided according to professional medical judgment and in accordance with ministry of health guidelines.”

The Israeli military referred queries to the foreign ministry, which referred them to the prison service.