Some of the migrants were injured, Eurotunnel France spokesperson Cecile Carreras told CNN. French authorities and Eurotunnel personnel were able to enter the tunnel and intervene
The tunnel, also known as the Chunnel, runs 50 kilometers (31 miles) from a point near Calais, in northern France to Folkestone, in southeastern England.
British Home Secretary Theresa May said France and Great Britain agreed to work together "to return migrants, particularly to West Africa, to ensure that people see that making this journey does not lead to them coming to Europe and being able to settle in Europe."
Calais has long been a gathering place for migrants trying to find a way into the UK. And the journey has been deadly.
Last Friday, a 23-year-old Eritrean woman died after being hit by a car as she was trying to make her way to the UK, said Gaetan Genel, spokesman for France's Nord Pas de Calais region.
The woman was the eighth migrant to die since June 1, Genel said.
Since her death, Pas de Calais administrator Fabienne Buccio announced new safety measures.
"The authorities will now work with migrants to inform them about the potential dangers they could face on the A16," Genel said, referring to the road leading to the tunnel.
"They need to be aware that they are putting their lives in danger."
Genel said Buccio also asked authorities to continue installing barriers and to install new lighting, which would allow drivers to see when migrants try to jump on their trucks at night.
May, the British home secretary, said both countries have invested heavily in tunnel security.
"In Calais, the French government has already been pushing in for extra resources and extra police resources. And the UK government will pushing up to 17 million pounds ($26.5 million) more to ensure the security of the Eurotunnel."
British Prime Minister David Cameron said officials have have invested money in the fencing around Calais.
"We are also adding fencing around the entrance to the tunnel in Coquelles," he said. "We are doing everything we can."
But French Interior Minister Bernard Cazeneuve said the problem extends beyond France and Britain.
"If we want to solve this problem in Calais, if we want to prevent the networks of smugglers from driving vulnerable men, women and kids to Calais, we need to work on this problem in its origin," he said.
"We need to work on this from the migrant's countries of origin and follow their path which leads to the European territory."