Barcelona's most famous street Las Ramblas was packed with tourists when a van drove into the crowds, leaving scenes of carnage.
Spanish emergency service claims Pakistanis among dead, injured in attacks
"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," Barcelona star Messi, 30, wrote on Instagram.
"We are not going to give up, there are many more of us who want to live in a world in peace, without hate and where respect and tolerance are the basis of coexistence," added the Argentine who arrived in the Catalan capital as a teenager 14 years ago.
13 dead, over 100 hurt in two Spanish seaside city attacks
Real Madrid's Portuguese captain Ronaldo took to social media to express his shock at the attack which also left more than 50 injured.
"Dismayed at the news coming from Barcelona. All the support and solidarity [to] the family and friends of the victims," he wrote on Twitter.
Like Ronaldo and Messi, other athletes took to social network in an outpouring of solidarity with the Catalan capital.
World voices solidarity with Spain after terror attack
"All my support for the families affected and the city," Spanish tennis ace Rafael Nadal wrote on Twitter, while his great friend, basketball player Pau Gasol, declared himself "devastated by what happened on the Ramblas in Barcelona. All my support for the victims, wounded and relatives in these difficult times."
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