Demolition teams are due to return to the French port of Calais to dismantle more makeshift shelters in the migrant camp known as the “Jungle”.
Overnight, riot police fired tear gas at migrants who were hurling stones at the demolition squads.
Authorities say the migrants must move to shipping containers on another part of the site.
But many fear this will require them to claim asylum in France, and give up their hopes of travelling to Britain.
The BBC’s Anna Holligan in Calais says that migrants, under cover of darkness, tried to access lorries on the motorway heading towards the port.
Riot police fired tear gas, forcing them back, she said.
Those living in the camp, mainly from the Middle East, Afghanistan and Africa, hope to cross the Channel to the UK, often using people traffickers to try to enter illegally.
French authorities believe about 1,000 migrants will be affected by the eviction plan while aid agencies say the number of people living there is much higher.
About 100 shacks were dismantled on Monday. Riot police fired tear gas after migrants began throwing boulders inside the camp. At least 12 shelters were set ablaze.
More trouble broke out later as groups of migrants fanned out across scrubland towards the motorway heading to the port.
French media say about 150 people, some wielding sticks and iron bars, walked on to the road to block vehicles.
Riot police pushed them back into the camp.
At least four people, including activists from the UK-based No Borders group, were arrested during Monday’s unrest, police say.
Earlier, Good Chance, a theatre group which works in the camp, said police were stopping volunteers from entering the camp.
French officials say public areas in the camp such as places of worship or schools will not be affected and describe the clearance as a “humanitarian operation”.
Conditions in the southern sector are squalid and the camp’s sprawling presence has become a controversial issue in both France and the UK.
The Jungle in numbers
- Total camp population is disputed – Calais officials say it houses 3,700, while Help Refugees puts it at 5,497
- Figures for the southern half (facing immediate eviction threat) are estimated at either 800-1,000 or 3,455
- There are 205 women and 651 children (423 unaccompanied), says Help Refugees
- Local government’s long-term aim is to have no more than 2,000 migrants living in Calais, says its chief, Fabienne Buccio
Officials say migrants can either move into converted containers in the northern sector of the camp, where there is room for 1,500 people, move to similar accommodation centres elsewhere in France or claim asylum in France.
But many residents are reluctant to leave the Calais area.
Afghan migrant Hayat Sirat told AP news agency: “Going to Britain… is what people [here] want. So destroying part of the jungle is not the solution.”