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.
Police said they had arrested a man but did not identify him or describe his role. Earlier they were searching for the van's driver who, according to local media, fled the scene on foot.
It was not immediately clear how many attackers were involved. La Vanguardia newspaper said one of the suspected perpetrators had been killed in a shootout with police on the outskirts of Barcelona.
Witnesses said 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.
Eyewitness Tom Gueller told the BBC: "I heard screams and a bit of a crash and then I just saw the crowd parting and this van going full pelt down the middle of the Ramblas and I immediately knew that it was a terrorist attack or something like that.
"It wasn't slowing down at all. It was just going straight through the middle of the crowds," he added.
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.
Prime Minister Mariano Rajoy said on Twitter he was en route to Barcelona. "Maximum coordination to arrest the attackers, reinforce security and attend to all those affected," he said.
U.S. President Donald Trump said: "The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help."
He added: "Be tough & strong, we love you!"
Mobile phone footage showed several bodies strewn along the Ramblas, some motionless. Paramedics and bystanders bent over them, treating them and trying to comfort those still conscious.
Around them, the boulevard was deserted, covered in rubbish and abandoned objects including hats, flip-flops, bags and a pram.
"We saw a white van collide with people. We saw people going flying," holidaymaker Ellen Vercamm told El Pais newspaper.
An eyewitness named Rebecca told La Vanguardia: "I've seen a lot of people knocked down on the floor and the people are running and crying. The van drove down the middle of the street dragging everyone with it."
Police sources told news agency Europa Press that one policeman was hurt by a car at a control point set up in Barcelona, although they could not immediately confirm if the incident was linked to the attack.
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.
French President Emmanuel Macron, whose nation has suffered some of Europe's deadliest militant attacks in recent years, tweeted: "All my thoughts and France's solidarity to the victims of the tragic attack in Barcelona."
A Vatican spokesman said Pope Francis was praying for the victims and wanted to express his closeness to all Spanish people, especially the victims and their families.
Witness Ethan Spibey told Britain's Sky News: "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."
Authorities in Vic, a small town outside Barcelona, said a van had been found there in connection with the attack. Spanish media had earlier reported that a second van had been hired as a getaway vehicle.
Barcelona is the capital of the wealthy northeastern region of Catalonia, which plans to hold a popular vote on Oct. 1 on whether it should secede from Spain. It is in dispute with the central government, which says the vote cannot go ahead because it is unconstitutional.
Following the attack, security staff at Barcelona airport suspended a strike that started in early August. "Our work is now more necessary than ever," a spokesman said.
It was the deadliest attack in Spain since March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.