Pro-Palestinian activists say they were subjected to severe abuse, including broken bones, humiliation and sexual assault, after being detained by Israeli authorities for taking part in a Gaza-bound aid flotilla last month.

The allegations, which Israel denies, have triggered multiple international investigations and growing diplomatic pressure.

France, Italy and Australia have opened inquiries following the detention of more than 430 activists from around the world involved in the latest attempt to break the blockade on the war-damaged Gaza Strip.

French nationals Meriem Hadjal, Noe Tissot and Malika Baouya were aboard the vessel Peluxo, which was carrying school supplies, infant formula and medical aid when Israeli speedboats intercepted it in international waters.

The activists said they were taken from the boat and violently herded together at sea onto what some described as a "torture prison ship.”

"I was dragged by the arm and lifted up with my hands tied behind my back. I screamed in pain. I thought my arm had been torn off,” said nurse Baouya.

"We walked with our heads down, hands behind our necks. We were made to lie on the floor in stagnant seawater. Men were tased,” she added.

Stripped to minimal clothing and fitted with numbered wristbands, the activists, backs bent and limbs shackled, say they were led one by one toward a dark container.

Afraid they would kill me

"When the door opened, I saw a fellow prisoner lying on the floor with his trousers down,” said Hadjal, 38.

"A soldier started groping my breasts. I was slapped hard. Then again. Some soldiers tried to push me toward the back of the container. I was afraid they would kill me.”

Baouya said she saw an activist on the ground being beaten before three men grabbed her.

One soldier "lifted me up by my hair,” while another "tried to rip off my underwear,” she said.

The Israel Defense Forces rejected the allegations, saying troops operate under strict standards of conduct and are required to treat detainees appropriately during operations involving the naval blockade.

An Israeli military official told Agence France-Presse (AFP) the flotilla members were under the custody of a special unit of the Israel Prison Service.

"A designated area was established with all of the necessities for the participants aboard the landing craft and secured by special Israel Prison Service personnel,” the official said.

The Israel Prison Service did not respond to AFP’s requests for comment.

Speaking to AFP in Melbourne, activist Violet Coco said soldiers laughed as they beat her, striking her in the head and kicking her repeatedly.

Her hand was injured as she tried to shield herself, she said.

"They were groping my private parts. I ended up with bruises on my breasts and other areas.”

The activists were confined for several days to a section of the ship’s deck surrounded by containers topped with barbed wire, shown in a widely criticized video released by Israel’s far-right National Security Minister Itamar Ben Gvir.

Baouya, who said she suffered a cervical spine fracture, was among "around a hundred others with disheveled hair and bloodied faces,” she said.

Hadjal, who said her foot was injured, said she saw another detainee "come out of the container with a swollen face, in a state of shock.”

The activists said they slept on freezing metal and wooden floors inside the containers, with limited water, food and hygiene, as seawater seeped in.

They also accused soldiers of firing stun grenades and rubber bullets at them.

Speaking out

The activists were later taken ashore in Israel and detained at Ktziot prison, where they said further abuse occurred, allegations the Israeli Prison Service denies.

Security personnel "were insulting us, making animal noises and hitting us with their rifle butts” upon arrival near the port, Tissot, 32, told France’s crimes against humanity unit.

Inside a tent, "a soldier landed a massive punch on my head and ribs,” cracking one, he said in his statement.

Back in Germany after his release, social worker Johannes Happel, 29, told AFP his head had been "slammed against a tent pole” and he saw another detainee being beaten and thrown to the ground.

"Cruel, sadistic and inhumane are the adjectives that spring to mind for everything I saw,” he said.

Another Australian activist, Neve O’Connor, described being forcefully removed from the boat and thrown onto a concrete floor.

"All you can hear is the Israeli national anthem being played on repeat,” she said. "It was so loud, you could hear your friends screaming.”

"What we experienced, even with our passports, is just a taste of what Palestinian prisoners go through,” said Hadjal, who described her testimony as "a weapon.”

Baouya, who is set to give evidence in the French investigation, said she and others were "speaking out not for ourselves, but for the Palestinians.”