French authorities began bulldozing half of the "Jungle" migrant camp in the northern port city of Calais on Monday, AFP reporters at the scene said.
Two bulldozers and around 20 workers began destroying makeshift shacks, with 30 police cars and two anti-riot vans stationed nearby.
The situation was mostly calm, although one member of the British "No Borders" activist group was arrested.
"We are carrying out our orders so that the migrants leave the camp and we will continue this work this morning... so that the destruction work can continue calmly and that the migrants are not under pressure from the No Borders activists," said local authority head Fabienne Buccio.
She accused No Borders activists of threatening workers on Friday, a day after a French court had given the green light to clear the southern half of the camp.