At least 1,200 Palestinian inmates of Israeli jails began an open-ended hunger strike on Tuesday, as rallies across the occupied territories marked "Prisoners' Day".
As thousands gathered in towns and cities in both the West Bank and the Gaza Strip, three-quarters of the 4,700 Palestinians held by Israel began refusing food, the Israel Prisons Service (IPS) said.
"In the framework of (Palestinian) Prisoners' Day, around 2,300 security prisoners said they were refusing their daily meals, and around 1,200 prisoners said they were starting a hunger strike," IPS spokeswoman Sivan Weizman said on Tuesday.
Another eight women inmates had also said they were refusing food as a show of solidarity with the Palestinian security prisoners, Weizman said.
"We have coped with hunger strikes in the past and we are prepared to do so again now," she added.
Speaking to crowds gathered in the northern West Bank city of Nablus, Qadura Fares, head of the Palestinian Prisoners Club, gave a higher figure for the hunger strike, saying "1,500 prisoners from all the factions" had joined it already and more were expected to later in the month.
"We are united and undivided when it comes to prisoners, and we will stand by them until they get their demands," he said.
The Palestinian Prisoner Affairs Ministry says there are about 4,700 Palestinians being held in Israeli prisons, including 319 being held without charge in so-called "administrative detention".
The action by prisoners comes on the same day that Palestinian activist Khader Adnan was due to be released as part of a deal struck earlier this year to end his 66-day hunger strike in protest at his detention without charge.
Throughout the morning, thousands of people held marches and rallies across the West Bank, with about 3,000 people gathering in Shuhada Square in central Nablus, waving Palestinian flags and holding up pictures of imprisoned relatives.
Another 1,000 or so people gathered in central Ramallah, with a sit-in planned for later in the afternoon outside the nearby Ofer prison.
In the southern West Bank city of Hebron, 1,500 people gathered holding up flags, pictures and slogans reading: "Stop the policy of solitary confinement."
Hundreds more gathered in the northern towns of Tulkarem and Qalqilya, a witneses said.
In Gaza City, about 2,000 people marched to the headquarters of the Red Cross where they set up a solidarity tent with the hunger strikers.
There are currently 10 Palestinians on hunger strike in Israeli prisons, four of whom have been transferred to prison hospitals because of the fragile state of their heath, the Palestinian Prisoners Club says.
Two of them, Bilal Diab, 27, and Thaer Halahla, 34, have been refusing food for 50 days, with medics expressing concern over their deteriorating health.
Another prisoner, Hassan Safdi, on hunger strike for 44 days, was being held in the same facility, with his condition described as "very serious".
All 10 are being held without charge under administrative detention orders, which means they can be held for renewable periods of up to six months.