From the world of cycling, the general director of the Vuelta, which begins Saturday in the French city of Nimes, said: "We are dismayed by the dramatic attack in Barcelona and our condemnation of these events is total".
A number of football stars who play in Spain posted messages on social media, while players from around the world also sent their condolences.
And Karim Benzema flicked up Marcelo's cross and turned defender Samuel Umtiti to fire home a second with Barcelona rarely looking like coming back.
Match of the Day presenter and former Barca player Gary Lineker took to Twitter by writing: "Deeply saddened to hear about the attack in the wonderful city of Barcelona".
The club also posted videos of a minute's silence held by players and coaching staff at the team's Camp Tito Vilanova training ground with similar memorials being held at academy grounds Ciutat Esportiva Joan Gamper and The Masia. "All my support to the city and families!"
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"Anger, sadness, helplessness. All my strength to those affected".
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The 32-year-old added: "Full support and solidarity with the family and friends of the victims".
Espanyol, the city of Barcelona's other top-flight team, tweeted: "The most lovely city in the world, our city".
Ronaldo enjoyed another remarkable season in which he inspired Real Madrid to a La Liga/Champions League double while Messi topped him in the Spanish goalscoring charts for Barcelona while going past a career landmark of 500 goals for the club. "My support and solidarity for the victims and families". "No to the dictatorship of fear".