Barcelona: Riot police engaged in a running battle with angry protesters outside Barcelona's airport on Monday after Spain's Supreme Court convicted 12 separatist leaders of illegally promoting the wealthy Catalonia region's independence and sentenced nine of them to prison.

A protester uses a fire extinguisher against police, not pictured, during a demonstration at El Prat airport, outskirts of Barcelona. Credit:AP

Officers fired foam bullets and used batons against thousands of protesters who converged on the airport after the verdict was announced in Madrid. Protesters fought back by throwing objects, spraying dark clouds with fire extinguishers, and breaking windows during clashes that lasted into the night.

Regional emergency service SEM said 75 people were treated for injuries at Josep Tarradellas Barcelona-El Prat Airport. Spain's airport operator, AENA, said at least 108 flights were cancelled.

Police also clashed with angry crowds late on Monday night in downtown Barcelona. They used batons, and sounds similar to the firing projectiles were heard.