Around 6,000 dockers have taken part in a protest in Strasbourg against plans to liberalise port services.
Dockers from across the EU fear they could lose their jobs if the proposals are approved by the European Parliament.
Protesters threw firecrackers, stones and metal missiles, smashing windows, causing extensive damage.
Police officials said only a few hundred protesters turned violent.
At least 13 dockers were arrested and 12 police officers were injured during the clashes.
Elsewhere, strike action, such as here in Pasajes, northern Spain, disrupted work at ports from Greece to Sweden.