Barcelona will wear black armbands for their match at the Camp Nou on Sunday following the attack in the city on Thursday in which a van plowed into pedestrians, causing at least 13 deaths.
The Catalan club will also hold a minute's silence before their league opener against Real Betis, as will all the teams in La Liga this weekend.
A club statement said: "FC Barcelona wants to express its profound sorrow and utter disgust at the terrorist attack that has hit the heart 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 the club's matches this weekend.
"On Sunday, before the 2017/18 La Liga opener against Betis, the Camp Nou will hold a minutes silence in recognition and memory of the victims.
Espanyol, the city's other top-flight club, condemned the violence and thanked those who had issued messages of support following the attack.
"Still dismayed by the terrible outrage that has occurred this afternoon in Barcelona as a result of a terrorist attack, RCD Espanyol expresses its deepest condemnation of this and all terrorist actions and sends its deepest condolences to all the relatives and friends of the victims of this lamentable episode," a statement said.
"With our hearts still shrinking to see 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 samples of support that has received both the city and our club from organisations, institutions and professionals in football and sport.
"Only together will we win the battle to horror. And this club is committed to help in everything to get Barcelona soon recovered from this terrible blow and return to show itself to the world as what it is, its most beautiful city and the pride of the whole espanyolista family and those who so much love it.
Both teams held a minute's silence before Friday morning's training sessions.
Barca players and staff united at the Tito Vilanova training ground while Espanyol held their final training session at their Dani Jarque base ahead of Saturday's game at Sevilla with a heavy heart.
Flags at the Camp Nou and Barcelona's training complex have also been flying at half-mast.
The news also had some effects on the transfer window in Spain as Villarreal postponed the presentation of new signing Carlos Bacca, while Barcelona cancelled a plan to introduce new futsal players.
"We can't go through with a celebration at this moment," said club president Fernando Roig. "We stand in solidarity with the victims. It's the moment to stand together. Carlos Bacca understands the postponing of his presentation. We will do it at a date in the future."
A wide array of football stars spoke out on social media earlier in the day, with Barcelona's Lionel Messi writing on Instagram: "I want to send my condolences and all my support to the families and friends of the victims of this terrible attack in our beloved Barcelona, as well as totally rejecting any act of violence.
"We're not going to give up, there are more of us who want to live in a world of peace, without hate and where respect and tolerance form the basis of our coexistence."
Quiero mandar mis condolencias y todo mi apoyo a las familias y amigos de las víctimas del terrible atentado en nuestra amada Barcelona, además de rechazar totalmente cualquier acto de violencia. No nos vamos a rendir, somos muchos más los que queremos vivir en un mundo en paz, sin odio y donde el respeto y la tolerancia sean las bases de la convivencia.
Striker Luis Suarez added: "Really shocked at what has happened in Barcelona. All my support goes out to the city and the families involved!"
Fellow Barcelona players Javier Mascherano, Lucas Digne, Sergi Roberto, Gerard Deulofeu and Samuel Umtiti, as well as women's team player Toni Duggan, also penned messages on Twitter.
❤️Barcelona 🙏— Lucas Digne (@LucasDigne) August 17, 2017
🙏🏾❤️Barcelona— Samuel Umtiti (@samumtiti) August 17, 2017
Quina tristesa, ràbia, impotència.. Molta força a tots els afectats !! Barcelona ❤️— Sergi Roberto (@SergiRoberto10) August 17, 2017
"Anger, sadness, helplessness... All my strength to those affected."
Tota la força del món BARCELONA ..— Gerard Deulofeu (@gerardeulofeu) August 17, 2017
"All the strength in the world BARCELONA"
Barcelona... My prayers & thoughts are with you 😔❤️🙏🏼— Toni Duggan (@toniduggan) August 17, 2017
Neymar, who expressed his affection for the city amid his recent departure for Paris Saint-Germain, wrote: "Really shocked at what has happened in Barcelona. All my support goes out to the city and the families involved!"
Ronaldinho, who won two league titles and the Ballon d'Or with the club, wrote: "Too sad to see the news coming from Barcelona. My support and solidarity for the victims and their families."
Triste demais ver as notícias que chegam de #Barcelona. Meu apoio e solidariedade para as vítimas e famílias. 🙏— Ronaldinho Gaúcho (@10Ronaldinho) August 17, 2017
"Indignation, frustration and sadness is what I feel at the moment. Peace for Barcelona. Peace for the world," added Espanyol winger Hernan Perez.
Players from Real Madrid, who defeated Barcelona to win the Spanish Super Cup a day earlier, also expressed their condolences, including Cristiano Ronaldo, Gareth Bale, Sergio Ramos, Marco Asensio and Dani Carvajal.
Ronaldo wrote: "Dismayed by events in Barcelona. All my support and solidarity with the victims and their friends."
Consternado com as notícias que chegam de Barcelona. Todo o apoio e solidariedade a família e amigos das vítimas.— Cristiano Ronaldo (@Cristiano) August 17, 2017
Thoughts are with everyone in Barcelona tonight 🙏🏼— Gareth Bale (@GarethBale11) August 17, 2017
Barcelona. 💔— Sergio Ramos (@SergioRamos) August 17, 2017
No al terrorismo. No a la dictadura del miedo.#StopTerrorism
"Barcelona. No to terrorism. No to the dictatorship of fear."
Consternado por el suceso en Barcelona, todo mi apoyo para los familiares de los fallecidos y heridos.🙏🙏— Dani Carvajal Ramos (@DaniCarvajal92) August 17, 2017
"Dismayed by what's happened in Barcelona. All my support for the families of the dead and the injured."
Todo mi apoyo para las victimas y familiares del atentando de Barcelona 😔 #BastaYA— Marco Asensio (@marcoasensio10) August 17, 2017
"All my support for the victims and families of the attack in Barcelona #EnoughNow""
Real Madrid also released an official statement on their website, saying: "Real Madrid express their deepest dismay at the attack suffered in the city of Barcelona and their solidarity with the victims and their families and friends, at the same time as wishing a speedy recovery to those injured."
Manchester City captain Vincent Kompany took a stronger approach than most, writing:
We need to create a country or find a planet where fanatics, nazis and bloodthirsty leaders can go sort out their differences. All losers..— Vincent Kompany (@VincentKompany) August 17, 2017