Dozens of migrants scrambled aboard a cross-Channel ferry late Saturday in the northern French port of Calais, sparking a vast overnight police sweep of the ship, officials said Sunday.

Some 100 migrants, trying to reach Britain, broke through the port's perimeter and "around 50 of them managed to get aboard a ferry" which had just arrived from Dover, in England, senior regional official Jean-Philippe Vennin told AFP.

The migrants clambered aboard the Danish-operated DFDS ferry 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.

"Forty-four migrants were caught and detained, and taken by bus to Calais police headquarters," Vennin said.

On Sunday morning, police were still searching for up to four migrants believed to be hiding aboard the ship.

Cross-Channel ferry traffic was delayed overnight with at least two ferries having to hold stations at sea before being allowed into port.