The 44 migrants entered the highly protected port area on Saturday night and used ladders to climb aboard a ferry that had arrived from Dover in Britain.
French police arrested the migrants who had climbed aboard a ferry while it was docked at the port of Calais in the night of Saturday to Sunday, a Pas-de-Calais prefecture official told Reuters.
The migrants are being held at the Calais police headquarters while police officials continued searching the ship on Sunday morning.
The prefecture and humanitarian organizations estimate that several hundreds of migrants are living in the Calais area, all hoping to find a way to get to Britain.