Around 200 migrants tried to get into the Channel Tunnel from the French side overnight, clashing with staff and police and forcing a temporary suspension of rail services, a spokesman for Eurotunnel said.
Rail services resumed on Saturday morning but traffic was slow due to continued safety checks, the company said.
The incident was the latest in a series involving migrants who are camped around the northern French port of Calais in the hope of reaching Britain.
The migrants broke into Eurotunnel's French terminal at 00H30 local time (2230 GMT), with 120 entering the tunnel before being intercepted by French police as far as 15 km inside the tunnel, the company and police said.
"Such a large group had no chance of reaching the UK, so this was clearly an organised attack aimed at drawing media attention to the desperate situation of the migrants who are stuck in Calais," Eurotunnel said in a statement.
Rail services resumed as of 0600 GMT, with passenger services operating in both directions, but were subject to some delays, Eurotunnel said.
Calais is one of the flashpoints in a much wider pan-European immigration crisis. European countries are struggling to agree on how best to tackle the huge inflows of people fleeing conflicts or poverty in the Middle East and Africa.
Migrants in makeshift camps outside Calais known as the "jungle" regularly try to reach Britain by hiding in lorries and trains.
A tightening of security at Calais' sea port in recent months prompted increased attempts to access the Channel Tunnel, leading to the deaths of several migrants.
France and Britain have pledged to step up security at the tunnel after incidents this summer caused tensions between the two EU member states.
