If sustained, the strike led by Marwan Barghouti, a Palestinian leader jailed during the second Palestinian uprising, would be the largest in recent years.
It could heighten Israeli-Palestinian tensions at a time of renewed US attempts to restart stalled peace negotiations. Israeli rule over the West Bank and east Jerusalem, territories captured during the 1967 war and claimed by the Palestinians as part of a future state, reaches the half- century mark in June.
Israeli officials said some 1,100 prisoners joined the strike yesterday.
Palestinian officials and activists put the number of hunger strikers at 1,300 and 1,500, respectively, saying it is difficult to get updates from inside the prisons.
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They said some 6,500 Palestinians are currently in Israeli lockups. Israel calls them security prisoners held for offenses ranging from stone throwing and membership in outlawed groups to carrying out attacks that killed or wounded Israelis. Several hundred are being detained without charges.
Barghouti, a leader of the 2000 Palestinian uprising, is serving five life terms after being convicted by an Israeli court of directing two shooting attacks and a bombing that killed five people, including three Israelis.
Barghouti, who disputed the court's jurisdiction and didn't mount a defense, has been in prison since 2002.
In an opinion piece published this week in The New York Times, Barghouti alleged that Palestinian prisoners and detainees in Israel "have suffered from torture, inhumane and degrading treatment, and medical negligence." He said he organized the hunger strike to fight back, after exhausting all other options.
"These are terrorists and incarcerated murderers who are getting exactly what the international law requires," he told Israel's Army Radio. "My policy is that you can't negotiate with prisoners such as these... There is no reason to give them additional conditions in addition to what they already receive.