PARIS -- Paris police detained 12 Greenpeace activists who dumped a truckload of coal at the doorstep of France's presidential palace on Wednesday, hours before a visit by German Chancellor Angela Merkel.

The activists also said the truck had two containers of nuclear waste with radioactive tritium inside them. While the water inside containers had far above-normal levels of radioactivity, it was not a threat to cleanup crews or police as long as it wasn't spilled, Greenpeace activists said.

Police blocked off the road and hauled off the activists, and cleanup crews with shovels quickly removed the coal from the street.

The publicity stunt was aimed to send a message to French President Francois Hollande and Merkel "to abandon energies that are considered dangerous -- coal and nuclear -- and to finally commit to a real energy transition," said Greenpeace activist Sebastien Blavier. The group wants European countries to commit to raising their percentage of renewable energy use to 45 per cent by 2030.

Currently, France gets at least two-thirds of its electricity from nuclear power -- one of the highest such proportions in the world. Germany, meanwhile, last year slightly increased its share of electricity generated from coal to about 45 per cent. Environmentalists have criticized the increasing use of coal, saying it is a "dirty" source because of the large amount of carbon dioxide released when it is burned.

The stunt was bound to raise new questions about security at sensitive sites in France. Greenpeace France has recently carried out acts including peaceful invasions of French nuclear sites in a bid to expose security dangers.

Associated Press writer Geir Moulson contributed from Berlin.