Fires are raging across parts of the "Jungle" migrant camp in Calais, three days into a French operation to demolish it.
It was unclear who set the dozens of fires overnight and in the morning.
The clearance began on Monday and about 4,000 migrants - out of some 7,000 - have been taken from the squalid camp to shelters around France.
The prefect of Pas-de-Calais said authorities now expected to finish the evacuation operation on Wednesday.
The camp has become a key symbol of Europe's migration crisis, with its residents desperate to reach the UK.
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The BBC's Simon Jones, at the camp, says huts were set on fire overnight on the main street leading into the camp, leaving them in ashes.
He says this may have been a last act of defiance from migrants who did not want to leave and did not want to see their shelters taken down by the authorities.
Many of the makeshift shops that had been set up were destroyed and a London bus used by a charity to help women and children was torched.
Exploding gas canisters are a threat and more police officers are now being deployed to the site.
The prefect of Pas-de-Calais, Fabienne Buccio, told BFMTV is was "a tradition among the migrant population to destroy their homes before leaving".
However, the Calais police commissioner said he had been told by migrants that the fires were started by activists.
Camp resident Mahmoud al-Saleh told Agence France-Presse: "There were several fires overnight. Every time one was put out, another would erupt. It was clearly intentional.
"The firefighters came late. For a long time it was just us, migrants and volunteers, fighting the fires."
Dorothy Sang, of the charity Save The Children, told the BBC: "We know that hundreds of children slept in the Jungle last night, under the bridge, while fires were burning around them.
"We know that lots of them ran. It's a really, really dangerous situation for children right now."
More migrants joined queues on Wednesday for buses to take them out of the camp, with the situation calmer than the jostling of the previous morning.
Crews had begun dismantling the Jungle with sledgehammers on Tuesday. Workers in hard hats and orange jumpsuits pulled down unoccupied tents and shacks.
The work was carried out mostly by hand, and in a low-key manner, as officials believed sending in bulldozers would send the wrong message.
By the end of Tuesday, about 3,000 migrants had been moved out on coaches to centres across France, while another 1,000 unaccompanied minors had been given accommodation in containers near the Jungle.
Our correspondent says the figures are hard to get a grip of. The 4,000 migrants may be accounted for, he says, but the camp had an estimated 6,000-8,000 residents and it is possible a large number have disappeared - either to squat or sleep rough around Calais or go to other towns of their own accord.
The fear is they will return to set up camp again once the clearance is over.
More than 1,200 police officers have been deployed for the clearance operation in Calais.
The French interior ministry said officers "might be forced to intervene" if there was unrest during the demolition.
What is the Jungle?
- The Jungle camp is near the port of Calais and close to the 31-mile Channel Tunnel
- Officially about 7,000 migrants live in the camp. The Help Refugees agency said the final population ahead of its demolition was 8,143
- The camp was halved in area earlier this year but the population continued to rise, and reports of violence have increased
- Many migrants attempt to hide themselves in cargo vehicles entering the Channel Tunnel
- The area has been hit by protests from both locals and truck operators
Last year more than one million migrants - many fleeing the civil war in Syria - arrived in Europe. Countries struggled to cope and division arose in the EU over how best to deal with resettling people.
An EU-Turkey pact to try to stop migrants crossing to Greece and moves by Balkan nations to close their borders have driven down the number of people using the so-called eastern Mediterranean route.
However, migrants from African countries such as Eritrea and Somalia as well as West African nations such as Nigeria and the Gambia are continuing to attempt the crossing from Libya to Italy.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.
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