Demolition teams have moved in to the French port of Calais to dismantle more makeshift shelters in the migrant camp known as the “Jungle”.
French police have warned that they will use force if the migrants refuse to move to nearby shipping containers.
But many migrants fear they will be required to claim asylum in France and give up hope of travelling to Britain.
Overnight, riot police fired tear gas at migrants who were hurling stones at the demolition squads.
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.
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.
About 100 shacks were dismantled on Monday. 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 such as places of worship or schools will not be affected. They 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.
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.”