CALAIS, France (Reuters) - Hundreds of migrants trying to make their way to Britain from Calais in northern France clashed with French riot police on Wednesday, Reuters witnesses and police said.

Police used tear-gas grenades and led charges to disperse migrants trying to make their way to the approach road for Calais port.

The clashes took place not far from where a British-financed wall is being built to seal off the road, where migrants frequently try to jump on trucks bound for Britain.

Earlier on Wednesday, former president Nicolas Sarkozy was in Calais campaigning to return to the presidency in an election in April, promising to be particularly tough on immigration.

Sarkozy, who signed an agreement in 2003 which effectively put the British border on mainland France, said during the visit it was not up to France "to be England's border guards", and called for the deal to be renegotiated.

Under the current arrangement, thousands of migrants fleeing war or poverty in the Middle East, Africa and Asia have built a shanty town near Calais known as "the Jungle", which French authorities are currently dismantling.

(Reporting by Pierre Savary; writing by Leigh Thomas; editing by Andrew Roche)