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.

Israeli forces arrested 430 people onboard 50 ships in international waters on Tuesday to halt the flotilla of volunteers trying to take aid supplies to the Gaza Strip.

The allegations of abuse will add to pressure on Israeli authorities to explain the treatment of the detainees, after video footage of the Israeli security minister mocking some of the activists in prison sparked an international outcry.

Italy said EU members were discussing imposing sanctions on the minister, Itamar Ben-Gvir.

“At least 15 cases of sexual assaults, including rape,” organisers of the Global Sumud Flotilla posted on the Telegram social media app. “Shot with rubber bullets at close range. Tens of people’s bones broken.

“While the world’s eye is trained on the suffering of our participants, we cannot emphasise enough that this is a mere glimpse of the brutality Israel imposes daily on Palestinian hostages.”

Luca Poggi, an Italian economist among those detained from the flotilla, told Reuters on his arrival in Rome: “We were stripped, thrown to the ground, kicked. Many of us were Tasered, some were sexually assaulted, and some were denied access to a lawyer.”

Prosecutors in Rome were investigating the possible crimes of kidnapping, torture and sexual assault and would hear testimony from activists who had returned to Italy, the Italian legal source said.

A German foreign ministry spokesperson said consular officials who met German activists on their arrival in Istanbul reported that a number had injuries and were undergoing medical checks.

Humane treatment of German nationals was an “absolute priority”, the spokesperson said, adding: “We naturally expect a full explanation, as some of the allegations that have been made are serious.”

Sabrina Charik, who helped organise the return of 37 French citizens from the flotilla, told Reuters that five French participants had been hospitalised in Turkey, some with broken ribs or fractured vertebrae. Some had made detailed accusations of sexual violence, including of rape, she said.

In an Instagram post by an activist group verified by Reuters, a French national, Adrien Jouan, showed bruises across his back and on his forearms.

Activists said some of the alleged abuse took place at sea after their interception by Israeli naval forces, and some after their arrest and imprisonment in Israel.

Activists from several European countries were expected to arrive home on flights from Turkey after they were deported from Israel on Thursday.

Spain’s foreign minister, José Manuel Albares, told reporters that 44 Spanish flotilla members were expected to arrive throughout Friday on flights from Istanbul to Madrid and Barcelona. Four of them had received medical treatment for their injuries, he added.

Western governments expressed their anger on Thursday after Ben-Gvir posted a video of himself mocking activists being pinned to the ground in a prison.

The Italian foreign minister, Antonio Tajani, said on the sidelines of the Nato meeting in Sweden that he was in touch with all his EU counterparts “so that there may be a quick decision to impose sanctions” on Ben-Gvir.