Barcelona and Real Madrid both held a minute's silence prior to training on Friday in recognition of the victims of the attack in the Catalan capital.
Thirteen people died and more than 100 were injured when a van drove into pedestrians on the crowded Las Ramblas street, in an incident that local police are treating as an act of terrorism.
Five terror suspects were later killed in a police operation in Cambrils, a resort town roughly 100 kilometres from Barcelona, in a second vehicle attack in which one woman reportedly died.
A number of players from Barca and Madrid, including Lionel Messi, Cristiano Ronaldo, Gerard Pique and Sergio Ramos expressed condolences on social media late on Thursday as tributes were posted by footballers and clubs across the world.
The Clasico rivals both paused prior to their training sessions ahead of the new LaLiga campaign, while Barca officials held a vigil outside the Camp Nou offices.
Barca president Josep Maria Bartomeu and directors Pau Vilanova and Maria Teixidor visited Plaza de Catalunya in the city centre, where thousands gathered to pay their respects.
FC Barcelona join the minute's silence. The President Bartomeu and Directors Vilanova and Teixidor represented the Club at Pl.Catalunya. August 18, 2017
Barca will hold a special commemoration before their LaLiga meeting with Real Betis at Camp Nou on Sunday, while the Royal Spanish Football Federation has suggested that a minute's silence is held prior to all fixtures in Spain's top two divisions this weekend.
United we are strong August 18, 2017
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