Barcelona's players observed a minute's silence at training on Friday following the terror attack in the city on Thursday.
Thirteen people were killed and dozens injured as a van drove into pedestrians on Las Ramblas in Barcelona. Authorities have warned that the death toll may rise.
The players of Barcelona subsequently took the time, ahead of training, to commemorate those killed.
Real Madrid, too, honoured the victims with a minute's silence of their own.
Both Lionel Messi and Neymar - the latter now of PSG but formerly of Barca - issued touching tributes to the victims on social media.
Taking to Facebook, Messi wrote: "I want to send my condolences and all my support to the families and friends of the victims of the terrible attack in our beloved Barcelona, in addition to totally rejecting any act of violence.
"We are not going to give up, we are many more who want to live in a world in peace, without hate and where respect and tolerance are the basis of coexistence."
Neymar, meanwhile, wrote on Twitter: "God will comfort the families. #PrayForBarcelona."