Barcelona - Despite an appeal by Spanish police not to circulate images of those injured by the attack, there have been some very graphic videos and images posted to social media this evening, as well as video which appears to depict the movements of armed police as they search for suspects. Residents of Barcelona, and social media users from further afield, have tried to combat the spread of these images with a seemingly bizarre tactic - flooding the #barcelona hashtag with images of cute kittens. It was a tactic employed by Belgian social media users in 2015. Police there asked citizens not to tweet about armed counter-terror operations being carried out around the country, and told people to stay indoors and not go near windows. Belgians reacted by using the #BrusselsLockdown hashtag to share pictures of their pets and make jokes. Social media reactions like this can often seem glib and superficial in the face of the events which have sparked them, but academics have argued that they can play a role in dealing with collective national trauma. Sadiq Khan, the mayor of London, has tweeted his condolences. He said: "My thoughts are with the victims of this barbaric terrorist attack in the great city of Barcelona and with their brave emergency services." He added that London stands with Barcelona.