Four inflatable boats containing fifteen Greenpeace activists were 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 female 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.
The Swedish coast guard is monitoring developments and has sent two of its boats to track the ship.