Riot police and migrants camped near the French port of Calais have clashed in overnight violence that aid workers say reflects the growing frustration of refugees' inability to smuggle themselves aboard trucks and trains bound for Britain. Calais police say officers monitoring the 6,000-resident camp east of the port city were pelted with rocks after midnight Monday and again Tuesday as migrants tried to block a neighboring highway that leads to the main ferry terminal. Aid workers said police fired tear gas canisters at rock-throwing crowds on the highway and in the camp.