Hundreds of Palestinians in Israeli jails launched a hunger strike on Monday following a call from leader and prominent prisoner Marwan Barghouti, a Palestinian Authority official said.
Issa Qaraqe, head of prisoners affairs for the Palestinian Authority, said that "around 1,300 Palestinian prisoners" were participating in the hunger strike and the number could rise.
The Palestinian Prisoners Club NGO put the number at 1,500.
Israeli prison service spokesman Assaf Librati said that 700 prisoners had announced on Sunday their intention to begin a hunger strike.
"We are checking this morning to see the number of prisoners actually striking as some of them said they would only observe a symbolic protest strike and then resume eating afterwards," he said.
"There will be an update later."
Barghouti is serving a life sentence over his role in the violent second Palestinian intifada. He is a popular figure, with polls suggesting he could win the Palestinian presidency.
The strike was called in connection with Palestinian Prisoners Day, which is observed annually.
Some 6,500 Palestinians are currently detained by Israel.