Barcelona will wear black armbands 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 opening match of the season 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 Thursday'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 80 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 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 matches this weekend.
"On Sunday, before the 2017/18 La Liga 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.
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."
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."