Shocking images were broadcasted out of Bilbao airport in northern Spain on Saturday following clashes with the police that broke out after activists from a Gaza-bound flotilla returned home.

According to Spanish media, four people were arrested for serious disobedience, resisting arrest and assaulting law enforcement officers, the Basque regional police force, the Ertzaintza, said in a statement.

"Following what happened at the airport, the Ertzaintza internal affairs division has launched an investigation to determine whether the officers' conduct complied with procedures," it added.