Protesting French sailors on Tuesday blocked access roads to the Channel Tunnel and the northern port of Calais, sparking traffic chaos, travel delays and diverted ferries.
Police broke up a sit-in on the A16 motorway leading up to the Channel Tunnel. Two protesters were hospitalised in the operation.
Demonstrators also burned tyres and crates, sending thick black smoke pluming into the air as tailbacks stretched several kilometres (miles).
Some of the thousands of illegal immigrants camped in Calais and desperately trying to cross the Channel to reach Britain took advantage of the chaos to climb onto stationery vehicles, said an AFP reporter on the scene.
The sailors, from the French ferry company MyFerryLink, were protesting plans to sell two of their ferries to rival firm DFDS.
"It's out of the question that DFDS takes our boats. Never, never," exclaimed the head of the main union on strike, Eric Vercoutre, who took up position with around 100 colleagues at the entrance to the tunnel in the early hours of Tuesday morning.
"We've been betrayed," he said.
A spokeswoman for Eurotunnel, which runs trains through the tunnel transporting cars, said that traffic leading up to the tunnel was "disrupted" but that actual transport through the tunnel was "relatively clear in both directions."
A handful of protesters also occupied the docks at Calais port. Two MyFerryLink boats and a P&O ferry were stuck in the port.
According to a source in the port, around 50 people prevented passengers from disembarking from a DFDS ferry from Britain in the early hours of Tuesday morning and the boat was forced to return to Dover in southern England.
Later ferries were being diverted to the port of Dunkirk.