The three activists who climbed onto the ship were taken into custody by police and brought to land for questioning, Swedish news agency TT reported.
The shipment was from Sweden’s first research reactor which shut down about two decades ago.
The ship left Studsvik in Sweden on Thursday, carrying 4.8 metric tons (5.3 tons) of spent nuclear fuel, including 1.2 kilograms (2.65 pounds) of plutonium for reprocessing at the Sellafield plant in northwest England. Once it has been treated in Sellafield, it will be returned to Sweden for final storage.
Sweden stopped shipments of nuclear waste to Sellafield in the early 1980s, and has since stored its waste at facilities in Sweden without reprocessing.
Greenpeace claims the Sellafield shipment is illegal and that the nuclear waste can be reprocessed in Sweden.
In a statement, Greenpeace campaigner Tarjei Haaland said the shipment broke Swedish radiation protection laws "and undermines 20 years of Swedish environmental policy not to reprocess spent nuclear fuel."
Swedish authorities have claimed that the plutonium will be returned in MOX (mixed oxide plutonium fuel) – but this is being challenged by Greenpeace. It claims that plutonium recovered from Magnox reprocessing cannot be processed into MOX fuel at Sellafield.
Greenpeace Sweden will also be challenging the return of PCM to Sweden instead of high level waste. It claims that as the UK has no facility for storing high level waste the deal contravenes Swedish radiation protection law.
Greenpeace Great Britain asks:
"...why are we importing other countries' waste into the UK anyway? The government has been very keen to stress the supposed benefits of new nuclear plants both in its original energy review and the re-run it's been forced to hold following our successful injunction. Conveniently though, there's been little or no mention of either nuclear waste disposal or reprocessing - the implication being that these issues are close to being solved, which is most definitely not the case.
There's loads of evidence that this shipment is illegal because it breaks Swedish laws regarding the export of nuclear wastes, so we have good grounds for our appeal against the initial decision to allow the shipment. In the meantime, we'll also be trying to get the governments of Norway and Denmark to put pressure on the Swedish government to keep the nuclear waste in Sweden and not dump it in the UK."
The following is from The Local (Sweden).
Environmental activists board nuclear waste ship
A ship carrying Swedish nuclear waste was intercepted by Greenpeace activists off the south coast of Sweden on Friday afternoon.
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.