Lionel Messi, who plays for Barcelona football club and is one of the world's most revered players, sent his condolences to the families and friends of the victims, saying he totally rejected "any act of violence," while Real Madrid's Cristiano Ronaldo and other world stars also offered their sympathy.
Barcelona and Real Madrid, Spain's biggest football clubs, both held a minute's silence before training on Friday.
The attack on Las Ramblas, a predominantly pedestrian zone in the heart of the Catalan capital, was the worst on Spanish soil since the 2004 Madrid bombings, which killed 191 people and injured more than 1,800.
A white van with blue markings accelerated into scores of people enjoying a late afternoon stroll and hours later in Cambrils, south of Barcelona, police shot dead five suspected terrorists.
Messi posted on Instagram: "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 rejecting totally any act of violence.
"We are not going to give up. We are many more who want to live in a peaceful world, without hate and where respect and tolerance form the base of coexistence."
Ronaldo, who plays for Barca's great rivals, said on Twitter that he was "dismayed by the news."
The 32-year-old added: "Full support and solidarity with the family and friends of the victims."
On Friday, Barcelona Football Club held a minute's silence at their training ground, posting a picture of the squad together with the hashtag #TotsSomBarcelona (we are all Barcelona).
The club tweeted on Thursday: "Deeply saddened by the attack on our city. All our thoughts are with the victims, their families and with the people of Barcelona."
In an additional statement, the Catalan club said: "FC Barcelona expresses its solidarity with the victims of the terrorist attack which has struck at the heart of our city, la Rambla de Barcelona."
Real Madrid, who had beaten Barca on Wednesday to win the Spanish Super Cup, issued a statement saying that the club "stands together" with the victims and their families and friends.
The statement read: "Real Madrid C. F. would like to express its deepest sorrow regarding the attack which has taken place in the city of Barcelona and the club stands together with the victims, their families and their friends.
"It also hopes for the quickest possible recovery to all those injured in the incident."
A number of football stars who play in Spain posted messages on social media, while players from around the world also sent their condolences.
Barcelona goalkeeper Marc ter Stegen tweeted: "We all need to be more united than ever."
Striker Luis Suarez tweeted his support to the city.
Manchester United striker Anthony Martial posted on Twitter.