Hundreds of prisoners are striking, including two men who have not eaten for 73 days. The strikers want better conditions and an end to an Israeli system of holding them for months without charge.
President Mahmoud Abbas told a West Bank rally on Thursday that if anyone is harmed, "we will not be quiet ever." He did not elaborate.
Palestinian activists say the men are in dire condition and may not survive much longer. Israel's prisons service says the men are under medical supervision and in stable condition. Officials have said some are taking liquids and mineral supplements, while others are refusing even that.