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Brief scuffles between police and protesters have broken out in Paris as thousands of people gather for May Day rallies under tight security measures.

Police used tear gas to control the crowd gathering near Paris' Montparnasse train station. AP reporters saw groups of hooded, black-clad people shouting anti-police slogans, mixing with other protesters wearing yellow vests or waving union flags.

French authorities have warned "radical activists" may join the Paris demonstration and renew scenes of violence that marked previous yellow vest protests and May Day demonstrations in the past two years.

More than 7,400 police are deployed in Paris.

Yellow vests have joined the traditional May Day union march to show their common rejection of French President Emmanuel Macron's economic policies.