The mass exodus came despite warnings from the United Nations and aid groups such a forced evacuation would have devastating humanitarian consequences, with hospital patients and others unable to move.

Israel has now extended a deadline for people to leave, allowing the safe movement of Gazans on two main roads south between 10am and 4pm local time (8am and 2pm UK time).

Follow live: Gazans told 'go south if you want to live'

As families in cars, lorries and donkey carts packed with their possessions headed south from Gaza City, Israeli airstrikes continued in the besieged territory.

Hamas said 70 people, mostly women and children, had been killed in an attack on a fleeing convoy.

Sky News has seen footage that appears to show the truck both before and in the aftermath of the strike.

It was not immediately clear who the target of the airstrikes was, or whether militants were among the passengers.

Meanwhile, the Israeli Defence Force (IDF) said its troops conducted raids into Gaza to battle insurgents and hunt for more than 120 civilians kidnapped in last weekend's shock assault by militants on southern Israel.

It is the first time Israel has stated ground troops have been operating inside the strip.

Israeli media reported remains of people who had gone missing in last week's attack had been found during the IDF raids.

The military also said a drone strike had killed a number of militants attempting to cross from Lebanon.

For nearly a week, Israel's military has been launching retaliatory airstrikes on Gaza targeting Hamas since the ruling Palestinian militant group stormed the border last weekend, killing hundreds of Israelis in their homes - as well as 260 others at a music festival.

The Palestinian health ministry said 2,269 people have been killed in Israeli strikes on Gaza, including hundreds of women and children, and thousands more injured.

Israel said a total of 1,300 of its people have been killed since Saturday's surprise raid as its troops continue to mass along the barbed wire fence ahead of a possible ground offensive on Gaza, with 360,000 reservists called up.

Palestinians had been ordered to evacuate to the southern part of the territory across the Wadi Gaza - a piece of coastal wetland with a river running through the middle - to "save their lives".

But Hamas militants have vowed to fight and told residents to stay, urging them "not to fall" for "fake propaganda".

Read more on this story:
Why Israel is braced for Hezbollah attack from Lebanon
How negotiators will be working to free Hamas hostages

The UN had appealed for the evacuation order to be rescinded to avoid turning "what is already a tragedy into a calamitous situation".

Early on Saturday, Israeli military spokesperson Lieutenant Colonel Jonathan Conricus said: "We have seen a significant movement of Palestinian civilians towards the south.

"Around the Gaza Strip, Israeli reserve soldiers in formation (are) getting ready for the next stage of operations.

"They are all around the Gaza Strip, in the south, in the centre and in the north, and they are preparing themselves for whatever target they get, whatever task."

While there has been strong support and sympathy for Israel over the Hamas attacks, the country's response has also sparked anger.

Pro-Palestinian protests are expected to be held around the world this weekend, including in London and other parts of the UK, fuelling concerns of potential flashpoints with counter-demonstrations in support of Israel.

A spokesperson for the Crown Prosecution Service said: "We have been working closely with the police ahead of planned protests in relation to recent events in the Middle East.

"The CPS will have additional staff on call this weekend who will provide round-the-clock charging advice if requested by the police.

"Individuals have the right to lawful protest but if any behaviour goes beyond that into an offence which meets our legal test, we will not hesitate to authorise criminal charges."

Elsewhere, efforts by the US to create a humanitarian corridor to allow the safe passage of civilians out of Gaza have been rejected by Israel's Arab neighbours.

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Egypt, the only Arab state to share a border with Gaza, and Jordan, which is next to the Israeli-occupied West Bank, have both warned against Palestinians being forced off their land where they want to build a future state.

The idea has echoes of the "Nakba", or "catastrophe", when many Palestinians fled or were driven from their homes during the 1948 war.

Egyptian President Abdel Fattah al Sisi said this week: "This is the cause of all causes, the cause of all Arabs.

"It is important that the (Palestinian) people remain steadfast and present on their land."