Lionel Messi and Cristiano Ronaldo are among the football stars to post messages of solidarity on social media following the terror attack in Barcelona on Thursday.
Catalonia interior minister Joaquim Forn confirmed that 13 people were killed and at least 50 injured when a van drove into pedestrians on Las Ramblas, a packed street in the city popular with tourists.
Local police confirmed two people had been arrested in connection with the incident, which they said they were treating as an act of terrorism.
Barca star Messi was among the first players to respond, writing via his official Instagram page: "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.
"We are not going to give up. There are many more of us who want to live in a world of peace, without hate and where respect and tolerance are the bases of living together."
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, defender Gerard Pique, Madrid forward Ronaldo and club captain Sergio Ramos also expressed their condolences.
"Dismayed at the news coming out of Barcelona. All support and solidarity to the families and friends of the victims," Ronaldo wrote.
There were similar messages from the official Twitter accounts of clubs across Spain and the world, including Barca, Madrid, Atletico Madrid, Manchester United and Paris Saint-Germain.
Neymar, who left Barca to join PSG in a world-record deal this month, tweeted: "May God comfort all the families. I love you Barcelona."
The Royal Spanish Football Federation has suggested a minute silence be held prior to matches in all domestic competitions over the coming weekend.Football stands with Barcelona after terror attack
|Chase hit with two-year ban for cocaine use|
|Jones returns to lead much-changed Wales against All Blacks|
|Chelsea´s best chance of success is Champions League, claims Lampard|
|F1 Raceweek: Hamilton not relishing imminent Halo, Vettel wants Baku awards|
|Cummins savours taking ´dream´ Root wicket|
|Jones makes nine England changes as Simmonds starts at number eight|
|Robben: It could be over for me after this season|
|Newcastle Jets 4 Melbourne Victory 1: Nabbout at the double|
|Chawrasia leads from Fitzpatrick in Hong Kong|
|Vince revels in silencing the doubters|
|Happy Sanchez could win the Premier League for Arsenal, Lauren claims|
|Davis leads Australian Open as Spieth struggles in the wind|
|Halep tips Dimitrov for grand slam glory|
|The Ashes round-up: England falter after Vince and Stoneman half-centuries|
|Angered Neymar storms away from Real Madrid questions|
|Bayern played their worst ´for a long time´ - Boateng|
|Australia fight back on tight first day in Brisbane|
|Neymar form no surprise for Alves|
|Fiji ready for another mission impossible - Naiqama|
|Vince helps England continue fine start|
|Thunder roll as Westbrook goes off against Warriors, Cavs win|
|Kiwis would consider Bennett as coach|
|Meninga unhappy with Aussie preparations ahead of World Cup semi|
|Federer congratulates Basel after Man Utd scalp|
|Heat snap Celtics´ 16-game winning streak|
|Spurs star Leonard has unique issue, Popovich says|
|England steady after early Cook loss in Brisbane|
|Gremio 1 Lanus 0: Cicero gives Brazilians advantage in Libertadores final|
|Hazard: I admire Madrid, but I´m happy at Chelsea|
|Neuer or De Gea? Digne says Ter Stegen is world´s best keeper|
|Allegri satisfied with draw against ´extraordinary´ Barcelona|
|Nainggolan reaffirms Roma commitment after Simeone praise|
|Chiefs sign veteran CB Revis|
|England choose to bat as Warner, Marsh cleared to play for Australia|
|Beathard, not Garoppolo, named 49ers starter v Seahawks|
|Simeone hails Griezmann´s ´extraordinary´ volley|
|Emery hails Neymar after stunning Celtic win|
|Freeney joins Lions day after being cut by Seahawks|
|Championship Review: Wolves thrash 10-man Leeds, Middlesbrough cruise|
|Goncharenko delighted for clean sheet star Akinfeev|
|I hope Barcelona rest Messi – Jesus|
|Broncos name Lynch starting QB for Raiders game|
|Thiago suffered a serious injury, says Heynckes|
|Valverde explains decision to bench Messi|
|Coleman the latest to join Rebels revolution|
|LeBron: Defensive consistency key to Cavs winning streak|
|Bills revert to Tyrod Taylor as starting QB after Nathan Peterman disaster|
|Absent Stokes wishes England well|
|Iniesta hails ´splendid´ Ter Stegen as Barcelona seal last-16 spot|
|Griezmann shrugs off goalless streak after wonderstrike keeps Atletico´s hopes alive|
|We should have been 5-0 up – Mourinho unimpressed with wasteful United|
|Toure reveals Guardiola anger with City´s Feyenoord display|
|Warriors star Durant says he will play against Thunder|
|Paris Saint-Germain set Champions League scoring record in Celtic rout|
|Sporting CP 3 Olympiacos 1: Dost on target as hosts stay in contention|
|Bielsa suspended by struggling Lille|
|Anderlecht 1 Bayern Munich 2: Lewandowski and Tolisso give visitors win|
|Atletico Madrid 2 Roma 0: Griezmann returns to form with wondergoal|
|Basel 1 Manchester United 0: Late Lang goal leaves Mourinho´s men with work to do|
|Paris Saint-Germain 7 Celtic 1: Cavani and Neymar score braces in sparkling win|
|Juventus 0 Barcelona 0: Messi benched but Valverde´s men in last 16|
|Bennett backs New Zealand to recover from World Cup exit|
|No Samoa donation from England squad - Borthwick|
|Conte pleased to take pressure off Atletico clash|
|Success in Europe will help Milan´s league form, Montella claims|
|Bauza sacked by Saudi Arabia|
|Browns´ Josh Gordon returns to practice|
|Azarenka confirms place at ASB Classic|
|Willian: For me, it was a penalty|
|Akinfeev keeps first Champions League clean sheet since November 2006|