Messi posted a strong response in the aftermath of a van mounting the pavement and mowing down civilians in a popular tourist location.
"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, as well as expressing my complete rejection of any acts of violence," wrote Messi on social media
"We won't give in, those of us who wish to live in a peaceful, hate-free world in which respect and tolerance form the basis of our coexistence are in the overwhelming majority."
The Blaugrana also paid tribute to those who lost their lives on Las Ramblas.
"Deeply saddened by the attack on our city. All our thoughts are with the victims, their families and with the people of Barcelona," stated the Blaugrana official Twitter account.
"Still shocked by the terrible hurricane that has taken place this afternoon in Barcelona as a result of a terrorist attack, RCD Espanyol expresses its deepest condemnation of it and all terrorist acts and sends its most sincere condolences to all relatives and friends of the victims of this unfortunate episode," it read.