Two British activists have said they were admitted to hospital after being beaten by Israeli forces who intercepted their Gaza aid flotilla last week.

Alice Chapman and Zak Khan were among 180 members of the Global Sumud flotilla detained by the Israel Defense Forces (IDF) in international waters near Crete late on Wednesday.

Khan, a Green party council election candidate, said he had been shot in the leg with a rubber bullet fired by an Israeli solder. “I was beaten by four people, repeatedly punched, kicked, spat on and accused of being a terrorist,” he said.

Chapman told the Guardian an Israeli solder had punched her.

Two members of the flotilla, Saif Abukeshek, a Spanish-Swedish national of Palestinian origin, and Thiago Ávila, a Brazilian national, are still being held in Israel. They have been brought before a court in the city of Ashkelon, but neither has been charged.

Khan said doctors had told him he had come close to suffering a broken jaw during his assault. He also contracted a chest infection from the conditions in which the detainees were kept onboard an Israeli prison ship.

According to Chapman, this involved about half of the detainees having to sleep inside shipping containers, while half had to stay outside. They endured severe cold at night and severe heat during the day, she said, adding that Israeli soldiers had denied them water during the day and used stun grenades to disrupt their sleep.

Several detainees were put in solitary confinement – some as a result of medical conditions – but Khan said others had been taken away and beaten up. One, whom he named as Richard, was “beaten very badly inside a container”. He said his fellow detainees were unable to see Richard, but that the beating was loud enough that they could hear it. “He was abducted due to essentially speaking loudly and, explicitly, for saying ‘free Palestine’,” he said.

The pair, who are now in Crete, said they were among 34 people who had been taken to hospital upon their release, three of whom had needed ambulances.

Israeli forces intercepted and detained the crews of at least 22 boats from the flotilla, which is attempting to break Israel’s maritime blockade of the Gaza Strip to deliver humanitarian aid.

Organisers said there were about 20 British nationals in the flotilla, of whom eight were detained. Two of the eight have arrived back in the UK, while the rest are spread across Crete and Turkey.

After it emerged that Abukeshek and Ávila had been taken to Israel for interrogation, Spain and Brazil issued a joint statement on Friday condemning what they described as the “abduction of two of their citizens in international waters by the government of Israel”.

The organisers of the Global Sumud flotilla condemned Israeli’s actions as piracy, saying those on board had been seized unlawfully more than 600 miles from Gaza.

Israeli authorities have been approached for comment.

The foreign ministry has previously dismissed the flotilla as a provocative “PR stunt” and insisted the country’s actions complied with international law.

The UK Foreign Office has not responded to a request for comment. It said on Thursday that it was engaging with Israeli authorities “with the expectation that the situation will be resolved safely and in line with international law”.

A spokesperson said: “Efforts to deliver aid by sea highlight the dire humanitarian situation in Gaza. Israel must do more to allow sufficient aid into Gaza, in line with agreed minimum targets.”