Israeli lawmakers began a fiery debate Monday over Prime Minister Ariel Sharon's plan to withdraw settlers and soldiers from the Gaza Strip, as thousands of demonstrators marched at the parliament and police beefed up security around Jerusalem.

Lawmakers have scheduled 17 hours of debate on Sharon's plan over Monday and Tuesday, with a vote scheduled for Tuesday night. Sharon is expected to win approval by a margin of about five votes in the 120-member Knesset, Israel's parliament.

If the plan is approved, more than 8,000 Israelis in 21 Gaza settlements would be forced to relocate in a process Sharon wants to begin early next year. In addition to the Gaza withdrawal, his disengagement plan calls for closing four settlements with about 470 residents in the northern West Bank.

Sharon opened the session in the Knesset with an impassioned speech acknowledging the departure from Gaza would be painful, but saying he was "absolutely convinced this disengagement will strengthen Israel" and promising not to waver in implementing it.

Repeatedly interrupted by heckling, Sharon accused some of his erstwhile allies in the settlement movement of being "messianic" extremists and called on Israelis to unite behind him and the Gaza withdrawal. Three members of the Knesset were evicted from the chamber during the speech for refusing to be silent.

Debate on the proposal opened as 16 Palestinians were killed and almost 100 wounded by the Israeli military in the southern Gaza Strip town of Khan Younis late Sunday and early Monday. An Israeli military spokeswoman said the latest operation targeted Palestinians who had fired more than 28 mortar shells at Jewish settlements and Israeli military targets in Gaza over the weekend. Palestinian militant groups said the attacks were in retaliation for the killing of a top leader of the Islamic Resistance Movement, or Hamas, in a targeted assassination by Israel Thursday night.

In a sign of how the Gaza issue has altered traditional political alliances, at least 17 members of Sharon's Likud Party -- including several cabinet ministers, and most of the ultranationalist and religious parties that typically are Sharon's staunchest supporters -- have said they will abstain or vote against the plan. At the same time, most of the dovish opposition in the Knesset, including the entire Labor Party, and some of Sharon's harshest critics, including several Israeli Arabs and well-known peace activists, have said they will vote with the prime minister to give him a majority.

But a victory for Sharon could come with a big price tag. Numerous Israeli politicians and political analysts claim that the issue could split the Likud into two parties, or cause the collapse of Sharon's government. Influential rabbis have called on Israeli soldiers to disobey orders to evacuate settlements, and there have been several death threats against Sharon.

The Gaza pullback would be the first withdrawal from territory seized in the 1967 Arab-Israeli war since Israel left the Sinai in 1982 following the Camp David peace agreement struck between Israeli Prime Minister Menachem Begin and Egyptian President Anwar Sadat.

In his speech, Sharon said his decision to advocate leaving Gaza was the toughest of his life, especially since he had been so deeply involved in promoting Jewish settlements there. But, he said, "We do not wish to rule forever over millions of Palestinians." There are about 1.3 million Palestinians in Gaza. After the withdrawals, about 240,000 Jewish settlers will remain in the West Bank, in 120 settlements, amid about 2.2 million Palestinians.

The pro- and anti-disengagement forces have launched emotionally charged campaigns to sway lawmakers, even though it became clear more than a week ago that Sharon had enough votes for his proposal to pass. The fervor of the anti-disengagement campaign reflected the possibility that the plan could be thrown out further down the legislative path or submitted to a nationwide referendum.

The pro-disengagement camp staged a torch-lit march outside the Knesset Monday night that drew thousands of demonstrators, and it ran a quarter-page ad in the Maariv newspaper declaring: "A fanatic minority will not decide the future of the state."

Anti-withdrawal forces inundated members of parliament with faxes, e-mails and telephone calls urging them to vote against the plan. On streets leading to the Knesset building, they hung signs with a crazed-looking photograph of Sharon asking: "Madman? Will the Egyptians maintain our security?"

Settlements across Gaza and the West Bank canceled school and ordered a strike on Tuesday -- the day of the vote -- so that thousands of children could be bused to the Knesset, where they would attempt to form a human chain around the building.

In the West Bank, meanwhile, officials said Yasser Arafat had undergone a diagnostic endoscopy and that doctors had found no major ailment after a week of concern about the Palestinian leader's health, the Reuters news agency reported.

The procedure was performed at Arafat's battered Ramallah headquarters, where he has been confined by Israel for more than two years.

Israeli peace activists, above, light torches before rallying at Israel's parliament building in support of Prime Minister Ariel Sharon's plan to pull troops and settlers from the Gaza Strip. Sharon, below, confers with officials during parliamentary debate on the proposal, which also triggered demonstrations by opponents.