Settlers formed human chains to try to block the troops at Neve Dekalim
At Neve Dekalim, the largest outpost, settlers, joined by Jewish hardliners from elsewhere, blocked the main gates, trying to keep the troops out.
About 9,000 people now have two days to leave all Gaza settlements and some in the West Bank or be forcibly removed.
More than 40,000 Israeli soldiers and police are part of the operation.
Compensation cut
This is the first time that Israel has agreed to dismantle settlements built on Palestinian land seized in the Six-Day War in 1967.
We don't know where to go. We don't know where our kids are going to school and it is two weeks before the 1 September and there's nothing prepared for us
Debbie Rosen, Neve Dekalim resident
Palestinian leader Mahmoud Abbas hailed the move as "historic" but said Israel should also pull out of the West Bank.
Hundreds of Palestinians marched through Gaza City to mark the start of the withdrawal and others headed for mosques to give thanks in a mass prayer meeting called by the militant group, Hamas.
Those living in Gaza's 21 Jewish settlements, and those in four of the West Bank's 120 settlements, have until midnight on Tuesday to leave.
Settlers who ignore the final deadline are to be forcibly removed and could lose up to a third of their compensation.
Tears from both sides
About half of Gaza's 8,500 settlers have already left. But those remaining have been joined by about 5,000 hardliners from Israel and the West Bank.
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At Neve Dekalim the unarmed soldiers delivering the notices were confronted by protesters calling on them to disobey their orders.
The BBC's Richard Miron at Neve Dekalim says this is a highly emotional moment for both sides. Many protesters were in tears, along with a few soldiers.
Settlers spoke of feeling betrayed by the Israeli government's decision to evict them.
"I think that all this process is immoral, all this plan is just a disaster for us and when my government said it would find a solution for every settler, we find it a lie, because we are more than 600 families with no solution," Debbie Rosen, one of the residents at Neve Dekalim, told the BBC.
"We don't know where to go. We don't know where our kids are going to school and it is two weeks before the 1 September and there's nothing prepared for us," she added.
In an apparent attempt to avoid violence, the army has said it will not deliver notices to settlements where residents have threatened resistance. Instead the notices are being sent by post.
Area sealed
The settlements have been sealed off to prevent any more protesters joining those resisting eviction.
There were scuffles with soldiers at several sites early on Monday. Some settlers burnt their belongings.
"I don't want to leave anything for the Palestinians, anything I own they could enjoy," Yaakov Mazal-Tari, a farmer at Rafiah Yam, told Reuters news agency.
Although the pullout as a whole has already been approved by the cabinet and parliament, ministers agreed last year to hold a final vote on the evacuation of each settlement or group of settlements.
Last week they gave approval for the three isolated Gaza settlements of Kfar Darom, Morag and Netzarim.
Palestinian security
In an interview with the BBC Mahmoud Abbas said that he believed the pullout would help to promote security and stability in the region.
"We see this as a historical moment, as Israel is leaving the settlements for the first time since the beginning of the Arab-Israeli conflict."
Some 7,500 Palestinian security forces have been deployed on the outskirts of the settlements themselves.
Their role is to forestall militant attacks on settlers, and prevent attempts by Palestinians to rush into the settlements.
Israeli troops have begun implementing the historic pullout plan from the Gaza Strip by delivering eviction notices to the remaining settlers. They now have 48 hours, beginning this morning at 00:00 (UTC+3) to withdraw from their settlements before the army removes them by force.
The Gaza Strip

Many settlers have vowed to resist their evacuation and promised fierce but nonviolent resistance. They have blocked the gate of Neve Dekalim, one of the largest settlements in the Strip, and called upon the soldiers to refuse their orders. "You're a partner to a crime," they screamed to army members.

Maj. Gen. Dan Harel, Israel's commander over the Gaza Strip, said the army will not waver in managing the pullout: "We will reach every settler, just as we have planned."

The IDF has sent about 55,000 troops to evacuate some 8,500 settlers.

The Israeli government believes that the withdrawal from the Gaza Strip is vital for the security of the country, although the decision to withdraw was very controversial, causing splits in Sharon's governing Likud party. Defence Minister Shaul Mofaz called it a "painful and difficult, but historic day".

In a televised prime time address, Israeli Prime Minister Ariel Sharon outlined his reasons for the withdrawal: "This act is essential for Israel. Believe me, the pain I feel with this act is the full realisation that we must do it. ... We cannot hold onto Gaza forever, more than a milion Palestinians live there ... crowded in refugee camps, poverty and hotbeds of hatred with no hope on the horizon." Sharon also said that if any terrorist attacks by Palestinians were to occur after the pull-out his government would deliever the "harshest response ever". But he also stated that if the Palestians would stretch out their hand in peace "we will respond with an olive branch."

The Palestinian Authority fully agrees with the Israeli actions. "We tell the Israeli people, `You have chosen the right path,'. Don't listen to the voices of the extremists who want a continuation of the occupation. I don't want — and I will not accept — any clashes with the army or the settlers," Palestinian President Mahmoud Abbas told Channel 10 TV. Abbas also insisted that Israel must hand over control over the West Bank and East Jerusalem as well in the not so far future.

The PA itself has deployed some 7,500 security to prevent attacks by Palestinian extremists.

The Gaza Strip has been partially occupied by Jewish settlers for 38 years. The average per capita Palestinian annual income there in 2001 was $625 U.S. dollars and there are extreme dietary deficiencies among the Palestinian people. Israel has given no assurances that there will be a lessening of trade restrictions on goods coming out of the Gaza strip.