French dock workers have stormed the port city of Le Havre and thrown smoke bombs into fountains, the Associated Press has reported.
The trouble comes amid widespread strike action over the country's controversial reforms to labour laws.
The unions are involved in a stand-off with the government over reforms which will make it easier for firms to hire and fire employees.
European Commission President Jean-Claude Juncker has conceded that migration from EU states is "one of the main reasons of Euroscepticism in the UK".
Mr Juncker is one of many global leaders in Japan, including Prime Minister David Cameron, for a two-day G7 summit.
ITV News' Deputy Political Editor Chris Ship is there.
Immigration minister James Brokenshire has said "net migration from outside of the EU and within the EU remains too high".
It comes as latest official figures reveal it reached 333,000 for the year ending December 2015 - the second highest level on record. Net migration from the EU was estimated to be 184,000.
"These figures underline that there are no quick fixes or simple solutions," Mr Brokenshire said.
"However, we remain committed to reforms across the whole of Government to bring migration down to sustainable levels, which is in the best interest of our country."
He also insisted "leaving the EU is absolutely no panacea or silver bullet whatever some may suggest".
Former London mayor Boris Johnson, a prominent member of the Vote Leave campaign, has said latest migration figures reflect the "scandal" of politicians' promises over immigration.
He told ITV News: "I think they show the scandal of politicians continually promising year after year that they can cut immigration to the tens of thousands when they have absolutely no control providing we remain within the European Union."
"It's only if we vote to leave on June 23 that we can take back control of our immigration from the EU 28 countries and have a sensible policy that is based on the real needs of the UK economy," he added.
The GMB union has called on the Government to "take responsibility" for the pensions of Tata Steel workers and urged against any change in laws.
Ministers are considering changes to pension laws, which would leave steelworkers worse off in retirement but still fare better than many employees in other pension schemes.