Demolition teams have moved in to start dismantling makeshift homes at the Calais migrant camp known as the Jungle.
ines 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.
It has been reported that tear gas was used against stone-throwers while work on bringing the camp down was underway.
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 b ut 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.
A spokesman for Doctors of the World said its medics have been unable to enter the camp as the entrance was being blocked by police. Basic supplies and care materials were being prepared for those evicted.
Its UK director, Leigh Daynes, said: "Dismantling a large part of the main Calais camp is irresponsible and inhuman. It completely ignores needs on the ground and will just traumatise people already close to the edge.
"It won't stop refugees coming to Calais nor make the thousands there already disappear. People will just be forced elsewhere to live in even more squalid conditions.
"We hope the police act proportionately, as our medics regularly treat refugees who've been injured, sometimes seriously, at their hands."
Elaine Oritz, of the Hummingbird Project, which has been supporting Calais migrants and refugees, also said volunteers were not being allowed on to the camp.
She said teams started arriving at the Jungle at around 8am, telling people that they had one hour to leave the southern section of the camp.
"The French government has said that no one should live in such inhumane conditions," she said.
"However, although conditions are bad, there is a structure and a protective aspect at the camp.
"By dispersing people elsewhere, it means all the protective factors given to the people there, including medical help, will be removed."
Human rights organisation Liberty said political leaders should not be "looking away" amid the plight of refugees at the Jungle, including unaccompanied children.
Its policy officer Rachel Robinson said: "Many of those in the camp have family here. The Government must allow their claims for asylum to be determined here and work to provide safe and legal routes to the UK for those in need of protection."