Riot police and migrants camped near Calais have clashed in overnight violence that aid workers said reflects the growing frustration of refugees' inability to make their way to Britain.

Calais police said officers monitoring the 6,000-resident camp east of the port city were pelted with rocks after midnight on Monday and again on Tuesday as migrants repeatedly tried to block a neighbouring highway that leads to the main ferry terminal.

They said 27 officers have suffered minor injuries, including 11 early on Tuesday.

Aid workers said police fired tear gas canisters at rock-throwing crowds on the highway and in the camp.

Migrants said sparks and heat from some canisters caused scorch damage to tents, but no serious injuries were reported as most campers sought safety inside their tents and shacks.

Tensions have been mounting since France started imposing tough new security measures, including 15ft (five-metre) high razor-topped fences and increased police patrols, to stop the flow of undocumented people from reaching Britain by boat or train.

Police said the latest overnight clashes lasted about five hours.

Campers said Muslim hard-liners in the camp were angered by news on Sunday of an anti-immigrant demonstration in the centre of Calais. However, that rally attracted barely 80 people.

Migrants and the foreign volunteers helping them in the camp say the rising night-time unrest chiefly reflects growing frustration, particularly among young single men given lowest priority for better housing.