Some 100 migrants, trying to reach Britain, broke through the port's perimeter late Saturday, senior regional official Jean-Philippe Vennin told AFP.
“Around 50 of them managed to get aboard a ferry”, the “Calais Seaways”, which had just arrived from Dover, in England, he said.
At least 46 had been apprehended by Sunday morning, but several more were still on an outside catwalk attached to the ship's funnel and firemen were attempting to get them to come down, Vennin said.
“Firemen initially managed to get close to them but were unable to talk them down, a second attempt is underway,” the official said, adding that the operation was made difficult by strong winds and rain.
The migrants had managed to clamber aboard the Danish-operated DFDS ferry at night by using a maintenance ladder at high tide, he added.
“Two of the migrants fell into the sea and were quickly rescued by firemen,” Vennin said.
Police oversaw the offloading of vehicles arriving from Britain from the ferry before carrying out a top-to-bottom search of the ship.
The detained migrants were taken by bus to Calais police headquarters, Vennin said.
Cross-Channel ferry traffic was delayed overnight with at least two ferries having to hold stations at sea before being allowed into port.
Regular services resumed Sunday morning while the migrant-occupied ferry was displaced overnight so as not to obstruct other traffic.