Jerusalem (CNN) -- About 1,500 Palestinian prisoners held in Israeli detention began an open-ended hunger strike Tuesday.
Their action came as Palestinians across the West Bank and Gaza gathered in various events to observe the annual Prisoners' Day, marking solidarity for the thousands of Palestinians in Israeli prisons.
The Palestinian Prisoner's Association Club, an inmate advocacy group, said 1,500 prisoners were taking part in the hunger strike and suggested that more from a range of Palestinian political factions were expected to follow.
"The Palestinian detainees who are on hunger strike are protesting Israel's treatment to prisoners inside the Israeli prisons," said Amani Sarahna, a club spokeswoman. "They are protesting Israel's policy of administrative detention and solitary confinement for prisoners for months at a time. They are protesting the arbitrary fines imposed on prisoners by the Israeli authority, the prevention of families, especially of those from Gaza."
Sarahna added that another 10 Palestinian prisoners had been waging hunger strikes of varying durations before Tuesday's mass effort. Those strikers are currently suffering from difficult health conditions, according a club press release.
A prisoner named Omar Abu Shallal, who has been on a hunger strike for 44 days, "is suffering from total faintness and is having difficulty sleeping in additions to headaches and back pains," the release said.
Other prisoners have endured longer strikes, such as Bilal Diab and Thaer Halahla, who are currently on their 50th day, the released noted. CNN cannot independently verify those accounts.
Administrative detention is a controversial Israeli practice that allows authorities to detain people indefinitely. There is no requirement to charge detainees held under this practice. Of the roughly 4,700 Palestinian prisoners in Israeli prisons, approximately 300 are in administrative detention.
Israeli authorities deny that Palestinian prisoners are mistreated and said all are handled according to strict regulations.
In a statement released Tuesday, the Israeli prisons authority said it expected about 1,200 prisoners to commence a hunger strike but added, "we have dealt with hunger strikes in the past and we are set up to do so now."
The mass protest follows two high-profile hunger strikes by Palestinian prisoners.
In February, Khader Adnan ended a 66-day strike after his sentence was commuted, and last month, female prisoner Hana Shalabi was deported to Gaza after refusing to eat for 44 days. Both Adnan and Shalabi were being held in administrative detention for suspected terrorist activity, according to the Israeli government.
Their hunger strikes became rallying cries for Palestinians, who staged multiple rallies of support in the West Bank and Gaza and launched a social media campaign to shed light on Israel's detention policies.
Palestinian Prisoners' Day also was being marked by rallies in the West Bank and Gaza, with thousands expected to turn out.
CNN's Kareem Khadder and Michael Schwartz contributed to this report.