Barcelona will wear black armbands for their opening match of the season this weekend in tribute to the victims of the terror attack in the city.
A minute's silence will also be held ahead of Barcelona's meeting with Real Betis at the Nou Camp on Sunday, as well as at matches across Spain.
The club expressed its "profound sorrow and utter disgust" at today's atrocity, with stars past and present sending messages of sympathy and support.
A van ploughed into pedestrians in the busy tourist area of Las Ramblas in the attack, in which at least 13 people died and around 100 were injured.
Barcelona said in a statement: "FC Barcelona wants to express its profound sorrow and utter disgust at the terrorist attack that has hit the hear of our city, la Rambla de Barcelona. The club wishes to send our support and thoughts to the victims, their families and friends as well as the people of Barcelona and its visitors.
"As a mark of respect, the flags will fly at half-mast at the club's installations and the players from both teams will wear black armbands in all matches this weekend.
"On Sunday, before the 2017/18 LaLiga opener against Betis, the Nou Camp will hold a minute's silence in recognition and memory of the victims."
The Spanish Football Federation issued a statement expressing its "total condemnation" of the attack and asking for a minute's silence to be held before matches across the divisions.
The likes of Lionel Messi, Luis Suarez and Neymar were among those to express their shock and sadness.
Messi said on Instagram: "I would like to send my condolences and all my support to the families and friends of the victims of the terrible attack on our beloved Barcelona, as well as totally reject any act of violence.
"We are not going to give up, we are many more who want to live in a world in peace, without hate and where respect and tolerance are the basis of coexistence."
Former Liverpool striker Suarez said on Twitter: "Very shocked for what happened in Barcelona. All my support to the city and families!"
Club captain Andres Iniesta tweeted: "Everyone is ashamed of what has happened and the news that continue to come out of our city. All my love."
Defender Gerard Pique said: "More than ever all together against this attack in our city. All my support to those affected by this barbarism. #Barcelona."
Neymar, who left Barcelona for Paris St Germain this summer in a world-record €222m move, tweeted: "May God comfort all families #PrayForBarcelona I love you BARCELONA."
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.
He 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 said on Twitter: "Deeply saddened to hear about the attack in the wonderful city of Barcelona. What purpose this barbarity serves is beyond comprehension."
Chelsea's former Barcelona midfielder Cesc Fabregas said: "No words to see what they have done in our beautiful city Barcelona. All united more than ever. #PrayForBarcelona."
The club's former captain Carles Puyol tweeted: "My condolences to the relatives of the victims and support to the injured in the attack in #Barcelona. Terror will never win."
Espanyol, the city's other top-flight ream, described the attack as a "terrible outrage".
The club added in a statement: "With our hearts still shrinking at seeing Barcelona, our beloved Barcelona, attacked vilely by terror and hate, we want to show our solidarity with all the people affected as well as to thank enormously all the support that has been received both by the city and our club from organisations, institutions and professionals in football and sport."
Barcelona's great rivals Real Madrid expressed their "deepest dismay" and their "solidarity with the victims and their families and friends" in a statement on their official website.
Real 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."
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."