Services from the port of Calais have been disrupted after 50 migrants boarded a ship, the city’s mayor Natacha Bouchart has said.
It is understood that about 50 migrants in Calais boarded a P&O ferry named the Spirit of Britain, while it was unloading.
Ferry company DFDS Seaways tweeted that the port had been closed due to a “migrant invasion”.
The incident following a demonstration in the port in favour of migrants.
P&O Ferries described it as a “security incident” and said passengers could expect a delay of between 90 and 120 minutes.
The demonstration was held in support of migrants and, according to AP news agency, attracted a crowd of 2,000 people.
Some of the protesters at the demonstration carried banners saying “refugees welcome here”. People from Britain were among the demonstrators.
A statement from the Port of Dover said: “The Port of Calais is currently experiencing migrant activity which has caused disruption to ferry services.
“Therefore services to and from Calais via the Port of Dover are affected, but DFDS Seaways services are still running to Dunkirk as normal.
“The Port of Dover remains open for business, but the duration of this disruption to services remains unknown.”
Thousands of migrants are currently living in camps known as “the Jungle” on the edge of Calais.
The population of the camp has risen steadily in recent weeks to about 2,500, including about 250 children, according to medical volunteer group, Medecins Sans Frontieres.
In August last year, the UK and France signed an agreement on new measures to try and alleviate the crisis, including a new command centre to help tackle trafficking gangs.
Meanwhile, Jeremy Corbyn has visited a migrant camp in northern France and spoken of the “dreadful conditions” there.
The Labour leader said conditions in the Grande-Synthe camp near Dunkirk would be a “disgrace anywhere”, and Britain should be “part of bringing European support to people”.
He said the long-term solution was to deal with the conflicts causing the migration crisis.
The situation in Calais is part of a wider migration crisis in Europe – caused largely by people fleeing war and oppression in countries such as Syria, Afghanistan and Eritrea.
On Friday French Prime Minister Manuel Valls warned that Europe’s migration crisis is putting the EU at grave risk.