The three Greenpeace activists, all of whom are women, were arrested for illegally boarding the Atlantic Osprey.
Four inflatable boats containing fifteen Greenpeace activists had been sent out from Ystad earlier in the day.
The proximity of the rubber boats to a fishing boat meant that the activists went undetected and three of them were able to climb aboard the ship.
The three Greenpeace members – from Denmark, Finland and Sweden – chained themselves to the Atlantic Osprey’s stern ramp.
“We are demanding that the Atlantic Osprey turns back to Studsvik with its highly dangerous cargo,” said Greenpeace spokesman Jan Isakson.
Police said that the situation on board the ship was calm when the arrests were made shortly before 7pm on Friday.