Messi, Ronaldo unite for Barcelona

Football icons and longtime rivals Lionel Messi and Cristiano Ronaldo joined Spanish sports stars around the world in expressing their grief after Thursday's terror attack in Barcelona left 13 people dead.

Barcelona's most famous street Las Ramblas was packed with tourists when a van drove into the crowds, leaving scenes of carnage.

"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, in addition to totally rejecting any act of violence," Messi wrote on Instagram.

"We are not going to give up, there are many more of us who want to live in a world in peace, without hate and where respect and 

"Dismayed at the news coming from Barcelona. All the support and solidarity (to) the family and friends of the victims," he wrote.

Other athletes also took to Twitter in an outpouring of solidarity.

"All my support for the families affected and the city," Spanish tennis ace Rafael Nadal wrote, while basketball player Pau Gasol, declared himself "devastated by what happened on the Ramblas in Barcelona."

All football clubs in the Spanish first and second divisions will observe a minute's silence in memory of the victims when the new La Liga season starts on Friday.

"All clubs will pay homage to the victims after the tragedy in Barcelona," Spanish league officials said in a statement.

Three-time world motorcycling champion Marc Marquez, also a Catalonia native, accompanied his message with an image of a circle formed with the motto "I love Barcelona".

Spain's previous No 1 goalkeeper Iker Casillas wrote: "What sadness, all my solidarity with the victims and their families. A lot of strength Barcelona."