Suspected migrant activity has suspended Channel Tunnel rail services between the UK and France, Eurostar said.
The train operator tweeted concerned passengers, who were left stranded both inside and outside the tunnel after services were halted, saying: "Trains are waiting to enter the tunnel as trespassers are on the tracks at Calais, the authorities are on site to help."
A later tweet to another passenger said, " The power has been switch off whilst the authorities deal with the trespassers."
Three trains carrying passengers from Paris to London, one train from London to Paris, and one service running from Brussels to London have been delayed due to the incident.
A spokesman for Eurostar said police were at the scene, but it was unclear how long delays would last.
He said: "We have trespassers on the French side of the tunnel which means that the traffic is blocked. Five trains are stuck on both sides. We are waiting for the police to clear the zone so that the trains and traffic can start again".
He added: "Because we don't know when the site will be cleared we really can't say anything about how long the tunnel delay will be. It's impossible for us to say right now."
The train operator said it was first notified of the disruption at around 8.45pm.
Passengers on social media have reported that migrants may have climbed on to the roofs of trains.
Simon Gentry tweeted: " On a Eurostar apparently "surrounded by migrants". Been asked to "listen out for people walking on the roof" while we wait for the police."
Conor Maples wrote: " Our Eurostar has been stopped outside Calais. Train manager told us there are migrants on the roof. Police are now here. Very very sad".