Revolting attack
"The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help," US President Donald Trump tweeted.
France's President Emmanuel Macron -- whose country has witnessed a series of bloody jihadist atrocities including a truck rampage in Nice in July 2016 that killed 86 people -- said his thoughts were with the victims of the "tragic attack".
A spokesman for German Chancellor Angela Merkel condemned the "revolting attack" and British Prime Minister Theresa May said on Twitter that London "stands with Spain against terror".
The Nice carnage and other assaults including the 2015 Paris attacks on nightspots in the city were claimed by the Islamic State.
In another deadly vehicle attack in December, 12 people were killed when a man driving a truck ploughed into a crowd at a Berlin Christmas market.
Spain has emerged as a potential target for jihadists, with extremist websites mentioning it for historical reasons, since much of its territory was once under Muslim rule.
The authorities in Spain -- the world's third biggest tourism destination -- generally remain discreet on the terror threat.
But they publicise every arrest of alleged jihadists, most of them detained for propaganda, recruitment for extremist groups or "glorifying terrorism."
According to the interior ministry, more than 180 "jihadist terrorists" have been arrested since 2015