Migrants massed in their thousands in Calais were taking advantage of wildcat strike action by French ferry workers by trying to board UK-bound lorries.
Truckers forced to slow or stop due to queuing traffic on approaches to the northern French port were faced with migrants attempting to clamber on board.
Migrants were seen walking by the sides of motorways looking for opportunities to jump on to lorries heading to the Port for Dover.
Tyres and hay bales were left burning at blockades created by striking MyFerryLink workers fearful that they will lose their jobs early next month.
Ferry services to and from the Port of Dover were affected by the strike, but Eurotunnel said their crossings were running to schedule, with two departures per hour.
The industrial action comes amid a worsening migrant situation near the Port of Calais where numbers camped there have swelled to more than 3,000 since April.
Aid workers have reported a "catastrophic" situation, with predictions that some 2,000 more migrants displaced from war-torn countries including Eritrea, Syria and Afghanistan could arrive over the summer.
A Port of Dover spokesman said: "We will continue to monitor the situation closely in liaison with our ferry partners and the Port of Calais in order to resume normal operations as soon as possible."
There were calls from P&O for the French authorities to intervene to ensure the free movement of goods and people across the EU.
Helen Deeble, chief executive of P&O Ferries, which operates between Dover and Calais, said: "Our services have been stopped in Calais as a result of the decision by Eurotunnel to charter and then sell two of their ships to DFDS, at short notice and with neither party appearing to engage constructively with the French sea-going and shore staff at MyFerryLink."
She went on: "While we have tremendous sympathy for the difficult situation of these employees who fear that their jobs will end on July 2, in disrupting the operations of the whole of the port of Calais, Eurotunnel continues to operate and DFDS sails into Dunkirk.
"As a large and long-standing employer in Calais, we are bearing the brunt of the disruption. We call upon the French authorities to ensure the free movement of goods and people across EU borders and to ensure the safety of staff and customers in and around the port of Calais."
As the migrant crisis worsens, truckers were being advised not to stop within about 60 miles (97km) of Calais, to stick with other drivers and make sure padlocks are always kept on vehicles.
Donald Armour, the Freight Transport Association's international manager, said: "We have heard that the strikers have set up a blockade on the A16 with burning tyres and hay bales, bringing the main motorway to a halt.
"It costs about £50 an hour to have a lorry stuck in traffic. If you're a company with half a dozen lorries, it adds up to a lot of money being lost.
"It's their worst nightmare and not a good day to be travelling."
With the worsening migrant situation in Calais, Mr Armour said the FTA was "really, really worried" about someone getting seriously hurt.
"There is a lot of fighting between the migrants who all want to be on the best part of the road to get on to the lorries," he said.
"We haven't had a fatality but it's not good. The situation is definitely worse than it has ever been."
The strike action started in Calais at around 3.50am. Port of Dover officials said the situation was being monitored closely.
Operation Stack - where freight traffic is held on the coastbound carriageway of the M20 in Kent - was implemented as queues built up on approaches to the British port.
A partial blockade of the Channel Tunnel terminal in Coquelles has been removed and Eurotunnel was clearing the backlog of traffic.
Kent Police said that while the Port of Calais remained closed, services from Dover to Dunkirk were unaffected and extra crossings were being arranged.