Police removed anti-nuclear protesters who had climbed onto a dockyard crane on Thursday to prevent a Russian ship from loading what they called dangerous nuclear waste at the north-western French port of Le Havre.
The environmental group Greenpeace said 20 of its supporters - 17 French nationals and 3 Swiss - were taken in for questioning after the protest, which lasted more than 12 hours.
"The export of uranium waste is an immense scandal. Thousands of tonnes leave every year by sea and across Russia, to be abandoned in Russia in sites which are among the most polluted in the planet," Greenpeace said.
The French nuclear energy firm Areva, responsible for the nuclear site from which the material came, dismissed Greenpeace's concerns that the cargo was unsafe for transport.
He said the cargo contained depleted uranium which was being sent to Russia for enrichment to make nuclear fuel. Depleted uranium was not highly radioactive and posed no risks, he said.