JERUSALEM, Oct 1 (Reuters) - Israel releases 87 jailed Palestinians on Monday in a bid to shore up President Mahmoud Abbas against rival Hamas Islamists and ahead of a U.S.-sponsored conference on Palestinian statehood.

The prisoners, all members of Abbas's embattled Fatah or smaller secular factions, will be bussed out of the desert stockade of Kitsyot by 7 a.m. (0500 GMT) to Israel's boundaries with the West Bank and Gaza Strip, the Prisons Service said.

The freeing of prisoners is highly emotive for Palestinians, who see their nearly 11,000 brethren held in Israeli jails as fighters for freedom from Israeli occupation. Many Israelis say such amnesties encourage violence by Palestinian militants.

Israeli Prime Minister Ehud Olmert, who is to meet Abbas later in the week to discuss their peacemaking agendas, said only prisoners "without blood on their hands" -- a reference to deadly attacks against the Jewish state -- and willing to sign a document renouncing violence would be freed.

Fatah lost control of Gaza in June to Hamas, which rejects peace efforts with Israel that include a Middle East conference planned for November under U.S. auspices. Palestinians are divided on whether the parley will bring them closer to statehood.

Abbas's office has welcomed the planned prisoner release but said more needed to be done on the issue. The last such gesture by Israel was on July 20, when some 250 prisoners, most of them from Fatah, went free.

Twenty-nine of the 87 prisoners on Monday's release roster come from Gaza and the others from the West Bank. Israel had been expected to release more than 100 Palestinian prisoners. Asked about the lower figure, political sources said several prisoners had not met criteria set by the security services.

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