French police have fired tear gas and water cannons on protesters defending migrants in Calais as the government prepares to shut down the city's notorious 'jungle' migrant camp.

The demonstrators - including activists from migrant support groups, a far-left presidential candidate and migrants themselves - defied a ban by authorities at Saturday's protest.

The protesters say they are expressing solidarity with the migrants, who face imminent expulsion from the camp.

Hundreds of people came in by bus from other cities and were met by police, who used tear gas to disperse them and keep them from marching into the city centre.

French president Francois Hollande has pledged to close the camp by the end of the year and transfer thousands of migrants around France while their asylum cases are examined.