* Palestinian prisoners' hunger strike began in April
* Israel says 65 prisoners being treated in hospital
* U.N. expresses concern (Adds Israeli cabinet approves bid to block prisoner releases)
RAMALLAH, June 8 (Reuters) - Shops were shuttered in the West Bank city of Ramallah on Sunday in solidarity with nearly 300 Palestinian prisoners on hunger strike against Israeli detention without trial.
Black-and-white flags bearing slogans such as "Freedom for Prisoners" and "Chains must be broken" flew in the streets of Ramallah, the Palestinian commercial capital.
In Hebron, also in the Israeli-occupied West Bank, dozens of Palestinian protesters marched in the streets in support of the hunger strikers.
The hunger strike was begun on April 24 by a group of 120 prisoners held under what Israel terms "administrative detention" - or incarceration without trial of Palestinians suspected of security offences.
They were later joined by 170 other inmates who also demanded that Israel abolish the procedure, which has drawn international criticism.
Israel's Prisons Service said 65 hunger strikers were being treated in hospitals, although none were in critical condition and all were conscious. A Palestinian lawyer who has visited some of the hospitalised inmates put the number of prisoners who had required hospital care at 100.
"The weight of striking prisoners has gone down by an average of 16 kilograms," Jawad Bolus told Reuters.
NO PARDONS FOR CONVICTED KILLERS
On Friday, a U.N. spokesman said Secretary-General Ban Ki-moon was concerned about "reports regarding the deteriorating health of Palestinian administrative detainees". Ban, the spokesman said, reiterated his long-standing position that they be charged or released without delay.
Israel argues administrative detentions are sometimes necessary to avoid court proceedings that could expose sensitive intelligence information or informants.
Lawyers who visited prisoners over the past several weeks said Israel had begun a dialogue with some of the hunger strikers' representatives but no progress had been made.
Palestinians regard those jailed by Israel as heroes in a struggle for statehood.
Israel says Palestinians involved in violent anti-Israeli activities are terrorists, and it is in the process of enacting a law aimed at blocking, in any future peace talks, the release of prisoners convicted of killing Israelis.
Legislation that would enable judges to declare convicted killers ineligible for presidential pardons was approved by the full Israeli cabinet on Sunday for submission to parliament, a month after a ministerial committee gave the bill the go-ahead.
Israeli presidents have largely rubber-stamped releases of Palestinian inmates agreed by Israeli governments over the years as part of peace talks or prisoner swaps for Israeli soldiers.
But the issue has stoked political controversy in Israel, which called off a release of 26 Palestinian prisoners last month in a move that contributed to the collapse of U.S.-brokered peace negotiations.
(Reporting by Nidal Almughrabi and Ali Sawafta; Editing by Jeffrey Heller and Raissa Kasolowsky)