Demolition teams have moved in to start dismantling makeshift homes at the Calais migrant camp known as the Jungle following a court ruling allowing mass evictions.
Lines of police vans gathered on the perimeter of the slum's southern section as migrants and refugees were told their only option now was to move.
A spokeswoman for the Help Refugees charity said police had blocked entrances to the sprawling camp, which is home to around 4,000 people.
She said some migrants and refugees were "bewildered and scared" as teams started tearing down tents and shacks on Monday morning.
French authorities want to relocate people from the squalid, rat-infested site to heated containers nearby or to centres around France.
The action came after a judge in Lille ruled last Thursday that a partial clearance should go ahead, apart from social spaces, including schools and places of worship.
Campaigners had called for a postponement to remove people from the slum, saying that there is not enough new accommodation for people to move to.
Save the Children said dismantling the camp will "make what was already an appalling situation for children even worse".
State authorities have said up to 1,000 people will be affected but aid workers say the figure is likely to be much higher.
Help Refugees said its own analysis revealed there were 3,455 people living in the affected area.
Save the Children said nearly 400 unaccompanied children who have fled war, poverty and persecution from countries including Syria, Afghanistan and Eritrea live on the site.
Some have criticised the decision to send in demolition teams, including Liberal Democrat leader Tim Farron, who said razing the camp was not the solution.
But British hauliers welcomed the court's judgment.
The Freight Transport Association (FTA) said disruption caused by migrants cost the UK freight industry an estimated £750,000 a day last year.
FTA officials said a solution needed to be found to protect the £89 billion worth of UK trade which passes through the cross-Channel ports annually.