WATCH: Barcelona and Espanyol hold minute's silence for terror victims, get support from Neymar and Ronaldo
Likes of Messi, Suarez and Iniesta also tweet about shock and dismay following incident.
Barcelona's players and coaching staff held a minute's silence before training on Friday in tribute to the victims of the terror attack in the city.
At least 13 people died and dozens were injured on Thursday when a van ploughed into pedestrians in Barcelona's busy tourist area of Las Ramblas.
Barca had already confirmed that players of all their teams would wear black armbands during games this weekend - the senior side begin their Primera Liga campaign against Real Betis at Camp Nou on Sunday - while a minute's silence will be held at matches across Spain.
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The Catalan club also showed their support for those affected by the attack on Friday morning, with players and staff standing in a semi-circle at the club's training ground while holding a minute's silence.
Barca tweeted a short video of the tribute along with the TotsSomBarcelona (we are all Barcelona) hashtag.
Earlier in the day, Twitter and Instagram accounts of the Catalan club as well as those of many players - current and former - were abuzz with posts expressing shock over the terror attacks that killed 13 people and injured many more.
Several players pledged their support to the city and its people, while also offering their condolences to those who lost their lives late on Thursday night.
Barcelona-based Espanyol published photographs of their squad linking arms in a circle to pay their respects ahead of Saturday's trip to Sevilla.
Barcelona terror attack: What we know
Spanish and European champions Real Madrid also tweeted a picture of their first-team squad holding a minute's silence at Valdebebas, with the likes of newly promoted Catalan club Girona, Granada, Levante and Real Mallorca posting similar images of their players and staff on their accounts.
Many clubs across Spain had already tweeted their support for Barcelona on Thursday evening.
Updated: August 18, 2017 04:07 PM