Human rights groups say 2,000 are on hunger strike against indefinite detention without charge and alleged ill-treatment

The number of Palestinian prisoners on hunger strike in Israeli jails has grown to 2,000, with more preparing to join the protest next week, according to human rights groups in the West Bank.

The Israeli prison service is taking punitive measures against hunger strikers, including solitary confinement, the confiscation of personal belongings, transfers and denial of family visits, say Palestinian organisations.

Seven prisoners have been transferred to a prison medical centre, including Tha'er Halahleh, 34, and Bilal Diab, 27, who by Thursday had been on hunger strike for 58 days. Their appeals against imprisonment without charge – known as administrative detention – were dismissed by a military court earlier this week.

The men's condition is rapidly deteriorating, according to Addameer, a prisoners' rights group. It expressed "grave concern that these hunger strikers are not receiving adequate healthcare … and that independent doctors are still being denied visits to them".

Administrative detention is one of the main issues behind the protest. More than 300 Palestinians – a 50% increase since last year – are being held without charge, trial or even being informed of accusations or evidence against them. Their term of imprisonment is determined by an Israeli military judge. Halahleh has been held for 22 months; Diab since last August.

Israel says administrative detention is a necessary security measure and that disclosing evidence could put intelligence-gathering or security operations at risk.

The prisoners are also protesting over the use of solitary confinement, denial of family visits and the treatment of sick detainees. "They also want to be treated with respect and dignity," said Shawan Jabarin of the human rights organisation al-Haq. "They want an end to middle-of-the-night checks, strip searches, humiliation and general ill-treatment. They are asking for humane treatment."

One female prisoner, Lina Jarbouni, who is serving 17 years for aiding a suicide bombing, was placed in solitary confinement after joining the hunger strike last week. Six other female prisoners have said they will begin refusing food from 1 May.

The Israeli prison service said most of the hunger strikers were affiliated with militant organisations, but denied the number of protesters was rising. "It is the same as it was at the beginning - 1,350," said a spokeswoman. "Those on hunger strike are separated from prisoners who are eating, and they don't get visitors." The revoking of privileges was a normal procedure in a prison protest, she added.

Meanwhile, the leader of a West Bank village protest movement was released on bail this week after more than a year in prison before the verdict in his military trial on 13 May. Bassem Tamimi, who has been recognised by the European Union as a "human rights defender", is accused of incitement and organising illegal demonstrations. He has previously spent around three years in administrative detention.

Palestinians had a duty to resist the Israeli occupation through popular peaceful protest, he told the Guardian after his release. "They have military superiority, but we have moral superiority," he said.

Under the terms of his bail he is not permitted to enter his home village of Nabi Saleh, which has been the scene of weekly protests against the expansion of a nearby Israeli settlement built within the village boundaries. The Israeli army routinely fires teargas, water cannon and rubber bullets at demonstrators.

The protests would continue despite regular night-time raids by the Israeli military and the arrest of at least 80 of Nabi Saleh's 500 inhabitants, Tamimi said.