Police fired tear gas and three officers were injured in clashes with migrants who tried to take advantage of a trffic jam to board lorries heading for the UK on Saturday, the authorities said.
French police said that their British counterparts had restricted traffic entering the port, despite warnings that thousands of vehicles would be arriving over the weekend.
"Our British colleagues had opened only nine out of 14 lanes this weekend though we expected 9,000 vehicles in the Eurotunnel and 7,500 at the port," local deputy police chief Jean-Philippe Vennin told the AFP news agency.
The lane shortages considerably slowed down traffic on the main A16 motorway and at about 2.30pm French time around 50 migrants "took advantage of the bottleneck to board a few trucks", he said.
There was no information as to whether any migrants were injured.
Migrants who successfully climbed onto a truck were chased away by the driver after being alerted by nearby motorists, a police source said.
The A16 motorway is the principal artery for freight and passengers heading for Britain either via the Channel Tunnel or Calais port.
The French government cleared the "Jungle" camp, which at its height was home to some 10,000 people living in dire conditions, last October but Calais remains a magnet for migrants, many from conflict zones in the Middle East, Asia and Africa, trying to go to the UK.