Police officers have had to remove a group of environmental activists who abseiled off a bridge in Germany to prevent a barge carrying highly radioactive nuclear waste being taken to a new storage site.

A boat with three containers carrying nuclear waste was being taken across the Neckar river in southwest Germany on Wednesday, from a closed nuclear power plant in Olbrigheim to an interim storage facility in Neckarwestheim, some 50 kilometers (31 miles) downstream.

But despite heavy police protection, including helicopters, patrol boats, and officers on the shore, the 12-hour journey ground to a halt when four activists abseiled off a bridge in the Bad Wimpfen area with a banner reading, "Prevention instead of relocation."

Despite having been asked several times to leave, the activists representing the Robin Wood environmental group refused to budge.

“We will stay as long as it takes,” the Local quoted a spokesperson from the group as saying.

But the boat eventually went on its way after officers removed two of the protesters.

More protesters were present on the river banks, also holding up banners. Jochen Stay from the Anti-Atom Organization told DW that moving the waste by river was "an act of irresponsibility beyond compare."

The state of Baden-Wurttemberg's Environment Ministry responded on Twitter that radiation measurements taken along the ship route had showed that the waste was being "perfectly transported" along the river.

The nuclear waste was being carried in Castor containers, a kind of steel barrel designed especially to safely store highly radioactive material. This was reported to be the first ever shipment of nuclear waste to be carried across Germany by boat. Energy provider EnBW said five more boat trips were planned over the next few weeks to remove the rest of the waste.

The German government plans to shut down all its nuclear power plants by 2022, but the question of where all the waste from them will go is still unresolved, according to DW.