"We decisively condemn this cruel and cynical crime against civilians," Putin wrote in a telegram of condolences to Spanish King Felipe VI.

"What happened once again confirms the need for a genuine unification of efforts by the entire world community in an uncompromising battle with the forces of terror," Putin said.

At least 13 people were killed Thursday when a driver deliberately slammed a van into crowds on Barcelona's most popular street in what police said was a "terror attack".

The attack, the latest in a wave of vehicle rammings across Europe in recent years, caused panic on the streets of Spain's largest city and drew condemnation from world leaders.

The famous Las Ramblas is one of Barcelona's busiest streets, lined with shops and restaurants and normally thronged with tourists and street performers until well into the night.