Around 5pm (1500 GMT), a vehicle slammed into a crowd of pedestrians on the famous Las Ramblas boulevard.
The promenade in the heart of the city centre is one of the city's busiest streets, normally thronged with tourists and street performers until well into the night.
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- Witnesses described scenes of chaos and panic as they tried to flee.
Who are the victims?
Regional interior minister Joaquim Forn said at least 13 people had died and more than 50 were injured in the attack.
Belgium's Foreign Minister Didier Reynders said on Twitter a Belgian national was among those killed in Barcelona. He told Belgian media the victim was a woman.
The Dutch Foreign Ministry also said in a statement that three Dutch citizens had been injured in the attack and were "in contact with their relatives".
"They are out of danger and have been taken to a hospital," the ministry said.
A Greek diplomat in the city said three nationals had been injured -- a woman and her two children.
Police in the Spanish region of Catalonia where Barcelona is located said on Twitter they have arrested one man and are treating the incident as a "terrorist attack".
He has been named as Driss Oukabir, originally from Marseilles in France and has been living in the Catalan city of Girona.
Initially a police source said one suspect had fled to a nearby bar, but this was later denied.
One witness told Spain's TVE television he saw the suspect when the van stopped.
"It was a person in their 20s, he is very young, brown hair, a slim face."
How did authorities respond?
Emergency services quickly arrived on the scene and cordoned off the area, with several ambulances and police vehicles responding.
The city also closed down metro stations in the area, with authorities telling people to stay away from the area.
Prime Minister Mariano Rajoy tweeted that he was in contact with the local authorities, saying the priority was to help the victims and facilitate the work of security forces.
Police appealed to people to stay in their homes and avoid unnecessary trips.
Previous attacks in Spain
Spain was hit by what is still Europe's deadliest jihadist attack in March 2004, when bombs exploded on commuter trains in Madrid, killing 191 people in an attack claimed by Al Qaeda-inspired extremists.
In July 2015, a hooded attacker opened fire outside a hotel in downtown Barcelona near Las Ramblas boulevard, leaving two people injured, police said.
No suspected motive for the attack was given.