The Port of Calais had to be temporarily closed after dozens of refugees and migrants stormed on to a ship in the hope of reaching the UK. About 50 people made their way on to a P&O-operated vessel called Spirit of Britain.
British anarchists were among some 35 people under arrest in Calais on Sunday after leading a 'scandalous' invasion of the ferry port by some 500 migrants.
Police attended the incident, which was resolved later in the evening and the port reopened. Services to and from Calais had been severely disrupted while it was dealt with.
The BBC quoted Calais mayor Natacha Bouchart as saying 50 asylum-seekers had boarded a P&O ferry while it was unloading. It said the police were on the scene.
A statement from the Port of Dover said services were disrupted due to "migrant activity" in Calais, with services in and out of Calais affected.
"This latest episode has made the headlines, but the many incidents of attacks and intimidation faced by our British drivers on a daily basis are going unreported as, depressingly, they are now being regarded as routine," said an observer.
In August, the UK and France reached an agreement aimed at stopping security breaches and easing the migrant crisis.