Hundreds of Palestinian prisoners staged a one-day hunger strike on Thursday in solidarity with others who have been fasting for two weeks in protest at being imprisoned without trial.
"About 90 have been refusing food for two weeks," Israel Prisons Service spokeswoman Sivan Weizman told AFP.
"Today several hundred more joined them but just for one day," she said.
Palestinian prisoners minister Issa Qaraqe said around 120 administrative detainees are on open-ended hunger strike.
"They are determined to continue their strike until their demand for the abolition of administrative detention is met," he told official Voice of Palestine radio.
About 5,000 Palestinians are being held in Israeli prisons, nearly 200 of them under administrative detention orders which allow suspects to be imprisoned without trial for up to six months.
Such orders can be renewed indefinitely by a military court.