Eurotunnel services have been suspended after a "massive invasion" of more than 100 migrants in the French terminal.
A "large and co-ordinated" group stormed the tracks at Calais at around 12.30am local time (11.30pm BST), Eurotunnel said.
A spokesman said: "It's a massive invasion and intrusion by a very large and co-ordinated group of migrants.
"They are being gathered up by the police authorities."
He added: "It's clearly an organised attack when it comes in such a large number, there are over 100 in this one group.
"They arrived together an in a well-organised manner broke through the fences and all clearly new where they were going."
British authorities are working with French police to clear the tracks.
It is expected to be hours before services are able to resume because of the large number of people on the tracks, Eurotunnel said.
"They are being gathered up by the police authorities and British police," a Eurotunnel spokesman added.
"It's a very co-ordinated and well-managed process but it's a very large group of people and it's dark so it will have to be done very carefully."
He added: "While they are on the track we have, for safety reasons, to suspend the service.
"Once that's done we will be able to restart."
Affected passengers are able to use their tickets on the ferry during the disruption.
"There's nobody (stuck) on trains, there are a small number of passengers affected and customers have gone to the ferry port to continue their journeys," Eurotunnel said.