Barcelona and Real Madrid have expressed their sadness at the terror attack in the Catalan capital in which up to 13 people are feared to have died and dozens more been injured.
A van ploughed into pedestrians in the busy tourist area of Las Ramblas on Thursday.
Barcelona said on Twitter: " Deeply saddened by the attack on our city. All our thoughts are with the victims, their families and with the people of Barcelona."
Their LaLiga rivals Madrid said in a statement on their official website: " Real Madrid show their deepest dismay at the attack in the city of Barcelona and express their solidarity with the victims and their families and friends and at the same time wish the injured a speedy recovery."
Real Madrid striker Cristiano Ronaldo, who was sent off in Monday's Supercopa first leg at the Nou Camp, added on Twitter: "Dismayed at the news coming from Barcelona. All the support and solidarity with the family and friends of the victims."
Ronaldinho won two league titles and the Champions League during a glittering five-year spell at the Nou Camp, during which he also won the Ballon d'Or.
The Brazilian tweeted: "Too sad to see the news coming from #Barcelona. My support and solidarity for the victims and families."
Former England striker Gary Lineker played for Barcelona for three seasons from 1986-89.
He tweeted: "Deeply saddened to hear about the attack in the wonderful city of Barcelona. What purpose this barbarity serves is beyond comprehension."
Manchester City, managed by former Barcelona midfielder and manager Pep Guardiola, said: "All at MCFC are saddened to hear of the attack in the City of Barcelona. Our thoughts are with all affected and the emergency services."
Spain's 15-time grand slam champion Rafael Nadal, whose uncle Miguel Angel Nadal played for Barca from 1991-99, said: "Shattered by what has happened in Barcelona! All my support for the families affected and the city."
Espanyol, the city of Barcelona's other top-flight team, tweeted: "The most beautiful city in the world, our city. We suffer with you and with all the people of Barcelona. Strength and encouragement, Barcelona!"
Atletico Madrid added: " Shocked by the terrible news coming from Barcelona; All our support for the victims and their families."
Serie A giants AC Milan tweeted: "Everyone at #ACMilan deeply regrets the attack in Barcelona. Our deepest condolences to all those affected."
Former Manchester United defender Rio Ferdinand said: " Shocking to see the news in Barcelona. My thoughts are with the victims & their families!"