Amnesty International urged Israel on Friday to either charge or release Bilal Kayed, a Palestinian on hunger strike for nearly two months in protest at his continued detention without trial.
"The Israeli authorities must release Kayed, or, if they have evidence that he has committed a crime, then he should be promptly charged with a recognisable criminal offence", a statement from the rights group read.
Kayed, whose hunger strike entered its 59th day on Friday, was moved from prison to Barzilai hospital in the southern Israeli city of Ashkelon after his health deteriorated.
He was currently refusing medical examinations and care and "is only consuming water, along with salts, sugar, and vitamin B1," according to Addameer, a Palestinian rights group.
The Physicians for Human Rights NGO says his vision is failing, he has difficulty standing and that doctors have warned he could be at risk of a stroke.
Palestinian officials say he also has kidney problems.
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Kayed was to have been released in June after serving a 14-and-a-half-year sentence for activities in the Popular Front for the Liberation of Palestine, labelled a terrorist organisation by Israel, the European Union and the United States.
Instead, Israeli authorities ordered that he remain in custody until further notice.
Kayed appealed against his detention, and a hearing is due to be held at Israel's high court on October 5.
On Tuesday, Arab and some Jewish supporters of Kayed picketed Barzilai hospital. A group of right-wing Israelis staged a counter-demonstration, and the two sides exchanged punches. Police arrested 10 Jews and three Arabs.
Israel says administrative detention allows authorities to hold suspects while continuing to gather evidence, while Palestinians, human rights groups and members of the international community have criticised the system.
Of more than 7,500 Palestinians currently in Israeli jails, about 700 are being held under administrative detention, Palestinian rights groups say.
Palestinians have regularly gone on hunger strike in protest at their detention.