The French authorities were attempting to contain the invasion and remove the migrants after the incident led to the suspension of all passengers and freight services through the tunnel.
A Eurotunnel spokesman described the latest incident at its Calais terminal as a "massive invasion and intrusion by a very large and co-ordinated group".
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 knew where they were going. They are being gathered up by the police authorities."
Eurotunnel said train services were suspended for safety reasons after people were spotted on the tracks, but no passengers were trapped on trains as a result of the incident.
"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," said the spokesman, adding: "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."