The group broke through fences and stormed the Channel Tunnel in the French terminal at around 12.30am, Eurotunnel said.
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The French police took control of the situation on site to prevent migrants crossing to the United Kingdom, according to Eurotunnel.
Eurotunnel said the tunnel had been closed as the security forces went in to try and track them down.
Services are not expected to resume until at least 10am United Kingdom time and passengers have been warned of lengthy delays.
Earlier suspension of services had been caused by a “co-ordinated group of migrants”, possibly attempting to get media attention, said the rail company.
The spokesman added: “They ran through the terminal, knocking a few staff to the ground and throwing stones at them”.
He added: “While they are on the track we have, for safety reasons, to suspend the service”.
“We were able to send a first Eurostar from England at 8:05am followed by an empty freight train”, the Eurostar spokeswoman said.
Services have been suspended but there are no trains stopped in the tunnel and passengers have been directed to use the ferry to carry on with their journey.
Extra security, including fencing, paid for by Britain, has been put in place, aimed at making it harder for migrants to get onto the platforms and trains.
A total of 13 migrants have died trying to reach Britain since June.
On Tuesday, it was reported that a 20-year-old Iraqi man had been found dead in a UK-bound lorry.
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Last week, a migrant in his twenties died after being hit by a shuttle bus at the Eurotunnel site near Calais.
