Catalan police said they have arrested 2 person linked to the incident.
A van ploughed into crowds in the Spanish city of Barcelona on Thursday and a regional official said 13 people were killed, in what police said they were treating as a terrorist attack.
Islamic State has claimed responsibility for the attack. "The perpetrators of the Barcelona attack are soldiers of the Islamic State and carried out the operation in response to calls for targeting coalition states," the group's Amaq news agency said said, referring to a United States-led coalition against the Sunni militant group.
Police said they had arrested two men linked to the attack in the heart of the city, but did not identify or describe their role. Earlier police were searching for the van's driver who, according to local media, fled the scene on foot.
Witnesses saying the van zigzagged down one of Barcelona's busiest tourist avenues, Las Ramblas, mowing down pedestrians and leaving bodies strewn across the ground.
A government official in the region of Catalonia, which includes Barcelona, said on Twitter that at least 50 people were injured.
Spanish media had also reported that two armed men were holed up in a bar in downtown Barcelona, and reported gunfire in the area, However, police later dismissed those reports.
Following the van crash, emergency services said people should not go to the area around Barcelona's Placa Catalunya, one of the city's main squares at the top of the famous Las Ramblas avenue, and requested the closure of nearby train and metro stations.
Prime Minister Mariano Rajoy said he was in contact with authorities, and the priority was to attend to the injured.
The incident took place at the height of the tourist season in Barcelona, which is one of Europe's top travel destinations with at least 11 million visitors a year.
While full details of the van incident were not immediately clear, vehicles have been used to ram into crowds in a series of militant attacks across Europe since July 2016, killing well over 100 people in Nice, Berlin, London and Stockholm.
Witness Ethan Spibey told Britain's Sky News: "All of sudden it was real chaos. People just started running screaming, there were loud bangs. People just started running into shops, there was a kind of mini-stampede where we were, down one of the alleyways." He said he had taken refuge with dozens of other people in a nearby church.
"They've locked the doors because I'm not sure whether the person who may have done it has actually been caught, so they've locked the doors and told people just to wait in here."
In recent weeks, threatening graffiti against tourists has appeared in Barcelona. In one video released under the slogan "tourism kills neighbourhoods", several hooded individuals stopped a tourist bus in Barcelona, slashed the tyres and spray-painted the windscreen.
The deadliest recent attack in Spain was in March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.
Related: In Pictures | Terror attack in Barcelona leaves at least 1 dead, over 30 injured
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 "Injured people being treated in Barcelona after a white van crashed into a crowd of people. (PTI)")
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