UK Prime Minister Theresa May says her thoughts are with the victims of the “terrible attack” in Barcelona.
“The UK stands with Spain against terror,” she added.
Officials in the city say at least 13 people were killed and 50 hurt when a van ploughed into pedestrians on the main street of Las Ramblas.
The Foreign Office has issued advice to tourists to “stay inside and away from Las Ramblas” and wait for police to say it is safe to move.
It said it was in contact with local authorities.
London Mayor Sadiq Khan tweeted his thoughts were “with the victims of this barbaric terrorist attack in the great city of Barcelona”.
“London stands with Barcelona against the evil of terrorism,” he added.
London has seen two similar vehicle attacks in recent months.