Among those arrested was the well-known French yachtsman Eugene Riguidel, whose sailboat was impounded by the police.

The activists were part of a flotilla sent out to protest the arrival of the nuclear waste from the United States to the French port of Cherbourg.

They were due to be freed on Monday, according to Greenpeace, which staged a protest in Cherbourg to demand their release.

Two vessels carrying a shipment of 140 kilograms (308 pounds) of plutonium from US weapons arsenals left North Carolina on September 20 and were expected to dock in Cherbourg over the weekend.

Greenpeace activists had sent out two ships, and a flotilla of a dozen smaller boats to protest their arrival.

A Greenpeace spokesman in Cherbourg said he now expected the shipment to arrive late Monday or early Tuesday.

The plutonium is to be taken to the French nuclear reconditioning station at La Hague, then sent to a facility in southern France to be recycled and eventually returned for civilian use in the United States.

The US government has said the plutonium was being transported by sea as a one-off exercise.

Greenpeace said the long distances of road transport involved constituted a "considerable" risk, not least because the cargo's containers could easily be opened by shoulder-launched rockets.

Cogema, the French state nuclear company, issued a statement rejecting the charges, saying the transport of plutonium was carried out with "all safety guarantees" and that the truck convoy would be unmarked to avoid attracting attention.

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