More than 1000 Palestinians held in Israeli jails have begun a mass hunger strike against detention conditions.
The action is being led by Marwan Barghouti, a Palestinian leader jailed by Israel for life for five murders.
Barghouti has been touted as a possible future successor to Palestinian Authority President Mahmoud Abbas.
Protests in support of the prisoners are being held in the occupied West Bank, with youths clashing with Israeli security forces in Bethlehem.
There are fears that the hunger strike - observed by 1187 detainees, according to Israel's prison service - could fuel tensions across the Palestinian areas.
The start of the strike coincides with the annual Palestinian Prisoners Day, when Palestinians remember detained friends and relatives.
The issue of Palestinians held in Israeli jails is an ongoing source of tension between the two sides.
Palestinians regard the detainees as political prisoners. Many have been convicted of attacks against Israelis and other offences.
Others are detained under so-called Administrative Detention, which allows suspects to be held without charge for six-month intervals.
There were about 7000 Palestinians held in Israeli jails by the end of last year, according to Palestinian prisoners' groups.