Three Australian activists say they have launched a hunger strike in Crete, after being left there by Israeli authorities following the interception of a flotilla attempting to transport aid to Gaza.

Ethan Floyd, Neve O’Connor and Zack Schofield – three of six Australians released after their ships were intercepted on Wednesday – said they and their colleagues were subjected to mistreatment while held for two days onboard an Israeli vessel.

Greek officials said 31 of the roughly 175 activists from the flotilla were taken to a hospital on Crete. Schofield said the three Australians have since been discharged from Sitia hospital but remain on the island.

Twenty-two vessels were intercepted off the coast of Crete on Wednesday evening while travelling as part of the Global Sumud flotilla, which left Italy on Monday.

Schofield, who spoke with Guardian Australia after his release, said the activists were held by Israel on a transport ship. He said the vessel had been retrofitted as a prison, with the main deck dominated by shipping containers surrounded by barbed wire.

Schofield alleged the protesters were subjected to violence by the Israeli forces, despite claims from Israel’s foreign minister that they were “taken off unharmed”.

“They took people into the fourth shipping container and beat them with the butts of their rifles and batons, and with their fists and their feet,” he said.

“I saw a man shot at point-blank range with a rubber bullet in the leg and in the back. A friend of mine who was in the American delegation told me he was dragged into that fourth shipping container and repeatedly kicked in the testicles, among many other places that they beat him.”

Schofield alleged he witnessed a young Colombian woman being repeatedly punched in the ribs by an IDF soldier.

Guardian Australia has not independently verified Schofield’s allegations.

He described the treatment he received himself on board as “mild violence”.

“I had two flash-bang grenades thrown directly at my feet as I was sitting opposite the entrance to the prison yard, and I had to duck out of the way before they exploded in my face,” he said.

“I was forced into stress positions, kneeling on the floor for lengths of time with my head slammed against the ground.”

He said crowded conditions meant about a quarter of the detainees were forced to sleep outside at any given time, and were flooded twice after Israeli soldiers pumped sea water over the deck.

The Guardian has sought comment from the IDF and the Israeli embassy in Australia.

Schofield, Floyd and O’Connor announced via video they had decided not to take any food from the Israelis “as they continued their starvation of the Palestinian people”, and until the Israeli authorities released two of the flotilla leaders, Thiago Ávila from Brazil and Saif Abu Keshek from Spain.

The Israeli foreign affairs ministry has confirmed Ávila and Abu Keshek were to be transported back to Israel “for questioning”.

“This Hamas-led-flotilla is another provocation designed to divert attention from Hamas’s refusal to disarm – and to serve the PR interests of professional provocateurs,” Israel’s foreign minister,” Gideon Sa’ar, posted on X on Friday.

“Saif Abu Keshek, suspected of affiliation with a terrorist organisation and Thiago Ávila suspected of illegal activity, will be brought to Israel for questioning. Israel will not allow the breach of the lawful naval blockade on Gaza.”

Flotilla organisers said the detention of the two men was illegal and asked international governments to pressure Israel for their immediate release.

Several European governments with nationals among those arrested have called on Israel to free the activists and called its action a flagrant contravention of international law.

Spain demanded the immediate release of Keshek, a Spanish national.

Sa’ar has defended the operation, posting on X that the IDF “successfully blocked attempts to breach the lawful naval blockade” and insisting that all participants “were taken off unharmed”.

Three other Australians – Bianca Webb-Pullman, Surya McEwen and Cameron Tribe – were also released.

The Australian Department of Foreign Affairs and Trade (Dfat) said consular staff were on the island to provide assistance.

“We are also continuing to liaise with local authorities in Israel and Greece including to confirm the detention of any Australians,” a Dfat spokesperson said in a statement.

“Australia has been part of the international call on Israel to comply with the binding orders of the International Court of Justice, including to enable the provision of basic services and humanitarian assistance at scale.

Supporters in Australia were planning a paddle-out event on Sydney Harbour on Sunday to show solidarity with the flotilla.

– with Agence France Presse