(Adds Palestinian call for Arab League meeting)
* Trump to unveil details of plan in Washington
* Israeli leaders briefed on contents of plan
* Gaza protesters say "Palestine not for sale"
* Palestinian leaders call for Arab League meeting
* (Adds Netanyahu indictment, and trip to Moscow)
By Nidal al-Mughrabi and Stephen Farrell
GAZA CITY/JERUSALEM, Jan 28 (Reuters) - Thousands of Palestinians demonstrated against U.S. President Donald Trump's Israeli-Palestinian peace plan on Tuesday hours before its scheduled release at a ceremony in Washington.
Israeli troops meanwhile reinforced positions near a flashpoint site between the Palestinian city of Ramallah and the Jewish settlement of Beit El in the Israeli-occupied West Bank.
While Israeli leaders have welcomed Trump's long-delayed plan, Palestinian leaders rejected it even before its official release. They say his administration is biased towards Israel.
The Palestinians fear Trump's blueprint will dash their hopes for an independent state in the West Bank, Gaza and East Jerusalem - areas Israel captured in the 1967 Middle East War - by permitting Israel to annex large chunks of occupied territory including blocs of Jewish settlements.
Diab Al-Louh, the Palestinians' ambassador to Egypt, said on Tuesday they had requested an urgent meeting of the Arab League council at ministerial level - which Palestinian President Mahmoud Abbas would attend.
In Gaza City on Tuesday, protesters waved Palestinian flags and held aloft posters of Abbas. "Trump is a fool, Palestine is not for sale!" an activist shouted through a loudspeaker.
Others chanted "Death to America" and "Death to Israel" as as they burned tyres and posters of Trump. More protests were expected after Trump announces details of his plan later in the day.
An Israeli military spokesman said troops had been sent to reinforce the West Bank's Jordan Valley - an area which Prime Minister Benjamin Netanyahu has pledged to partially annex.
Husam Zomlot, head of the Palestinian mission to Britain, told Reuters in London that Trump's peace plan was merely "political theatre".
"It is not a peace deal. It is the 'bantustan-isation' of the people of Palestine and the land of Palestine. We will be turned into bantustans," he said, refering to the nominally independent black enclaves in apartheid-era South Africa.
"Jan. 28, 2020 will mark the official legal stamp of approval of the United States for Israel to implement a full-fledged apartheid system," he said.
Israel vehemently rejects any comparison to the former South African regime.
Trump will deliver joint remarks with Netanyahu at the White House later on Tuesday to outline his plan, the result of three years work by his senior adviser and son-in-law Jared Kushner.
He met with Netanyahu and the Israeli opposition leader Benny Gantz ahead of the announcement. Both were briefed on its contents.
Netanyahu said it was "the opportunity of a century and we're not going to pass it by." Gantz called it a "significant and historic milestone."
A Netanyahu spokesman said he would fly to Moscow on Wednesday to brief Russian President Vladimir Putin on the proposals.
But Israeli-Palestinian talks broke down in 2014, and it is far from clear that the Trump plan will resuscitate them.
Palestinian and Arab sources who were briefed on a draft of the plan fear that it will seek to bribe Palestinians into accepting Israeli occupation, in what could be a prelude to Israel annexing about half of the West Bank.
Further obstacles include the continued expansion of Israeli settlements on occupied land and the rise to power in Gaza of the Islamist movement Hamas, which is formally committed to Israel's destruction.
Palestinian leaders say they were not invited to Washington, and that no plan can work without them. An Abbas spokesman urged any Arab or Muslim officials invited to the ceremony to boycott it.
Addressing their fears, Trump said on Monday: "They probably won't want it initially...but I think in the end they will. It's very good for them. In fact it's overly good to them."
But on Monday Abbas said he would not agree to any deal that did not secure a two-state solution. That formula, the basis for many years of frustrated international peace efforts, envisages Israel co-existing with a Palestinian state.
Palestinians have refused to deal with the Trump administration in protest at such pro-Israeli policies as its moving the U.S. Embassy in Israel from Tel Aviv to Jerusalem, whose eastern half the Palestinians seek for a future capital.
The Trump administration in November reversed decades of U.S. policy when Secretary of State Mike Pompeo said Washington no longer regarded the settlements on West Bank land as a breach of international law. Palestinians and most countries view the settlements as illegal, which Israel disputes.
Jon Alterman, director of the Middle East program at the Center for Strategic and International Studies in Washington, said both Trump and Netanyahu were looking to change the subject from their own domestic troubles.
"The problem is it doesn't feel like this is the beginning of an important initiative," Alterman said.
Trump was impeached in the House of Representatives last month and is on trial in the Senate on abuse of power charges.
On Tuesday Netanyahu was formally indicted in court on corruption charges, after he withdrew his bid for parliamentary immunity from prosecution.
Both men deny any wrongdoing. (Additional reporting by Ali Sawafta in Ramallah, Ari Rabinovitch in Jerusalem and Dan Williams and Steven Holland in Washington; Editing by Angus Macswan and Mark Heinrich)
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