About 100 people protested near the plant behind a large banner declaring "Stop plutonium". But they kept their distance, as did other protesters during the 1,000-km (660-mile) trip from northern France where the shipment arrived on Wednesday.

Helicopters circled over the trucks as it made its way from a nuclear reprocessing plant in La Hague in northern France and armed guards were placed on crossroads and bridges.

French state-owned nuclear energy firm Areva, whose Cogema unit will recycle the plutonium into nuclear fuel, declined to confirm the content of the convoy although a Reuters photographer saw it leave the La Hague plant early on Thursday.

Environmental activists said the shipment, which also made a more than two-week journey by sea from Charleston in the United States, was unsafe.

After several days of demonstrations before the shipment arrived, they staged only low-key protests after a court barred protesters from going within 100 metres (328 feet) of the shipment.

U.S. and French nuclear energy officials said security was tight and that all safety precautions had been taken.

Cogema will recycle the plutonium into nuclear fuel which will then be shipped back to the United States for use in an electricity-generating reactor.

It is part of the U.S. Department of Energy's international non-proliferation programme to turn plutonium from "excess" nuclear warheads into mixed-oxide (MOX) plutonium-uranium enriched fuel.

Greenpeace says the shipment is of 140 kg (308 lb) of plutonium. A spokesman for the U.S. Security Administration said the amount being transported is 125 kg.

The delivery is part of a post-Cold War agreement between the United States and Russia to get rid of plutonium from excess nuclear warheads.