At least 13 people have died and 80 were injured after a van ploughed into crowds in Barcelona’s famous Las Ramblas tourist area, police and local officials say.
The vehicle sped along the pedestrian area, mowing down people and sending others fleeing for cover.
Police are treating it as a terrorist attack.
The head of the Catalan region said two people had been arrested.
Spanish media are reporting another suspect died in a shootout. El Mundo newspaper said the shootout happened in Sant Just Lesvern, on the outskirts of the city.
What we know so far
Aftermath in pictures
Police have released a photo of a man named as Driss Oubakir, who is alleged to have rented the van used to drive into pedestrians.
Local media say he is in his 20s, and was born in Morocco. It is unclear if he is one of those arrested.
Police released this photo of a man alleged to have rented the van
So-called Islamic State has said it was behind the van attack in Las Ramblas, saying in a statement carried by its Amaq news agency that it was carried out by “Islamic State soldiers”. The group gave no further evidence or details to back this claim.
The attack happened at the height of the season in one of Europe’s top tourist destinations.
Witnesses said the van had deliberately targeted people, weaving from side to side as it drove down the boulevard.
Spanish media reports said the driver of the vehicle – a white Fiat that had been rented in the city – then fled on foot.
Vehicles have been used to ram into crowds in a series of attacks across Europe since July last year.
At the scene: By Anna Lindsay, BBC News
As I was trying to walk to Las Ramblas I was stopped nearby at a police cordon in the tourist melting pot of beautiful, palm-filled Placa Reial.
From there, I could see armoured vehicles lining Las Ramblas and ambulances screeching to get past, a sense of panic in the air.
Soon, several plain-clothed officers in bullet-proof vests shouted at everyone to immediately evacuate the square.
Visibly stunned tourists were left wondering how this could happen in what many consider one of the safest places to visit in Europe. Local residents could not believe this was happening in their city too.
We ran to a nearby restaurant, extreme nervousness and anxiety on the faces of everyone there, getting updates from the TV inside and mobile phones ringing endlessly.
The owner told us the police wanted him to close his premises immediately and we had to seek shelter elsewhere. The security zone was being widened as the attacker was being hunted, he said.
What did people see?
Tom Markwell from New Orleans, who was just arriving in a taxi in Las Ramblas, said: “I heard a crowd screaming. It sounded like they were screaming for a movie star.
“I saw the van. It had already been busted on the front. It was weaving left and right, trying to hit people as fast as possible. There were people lying on the ground.”
Aamer Anwar said he was walking down Las Ramblas, which was “jam-packed” with tourists.
“All of a sudden, I just sort of heard a crashing noise and the whole street just started to run, screaming. I saw a woman right next to me screaming for her kids,” he told Sky News.
“Police were very, very quickly there, police officers with guns, batons, everywhere. Then the whole street started getting pushed back.
“Police officers who got there just started screaming at people to move back, move back.”
Kevin Kwast, who is on holiday in Barcelona with his family, said: “I was eating with my family in La Boqueria market very near where the crash occurred.
“Hundreds of people started stampeding through the market… we started running with them going outside right into where casualties were already on the ground.
“Police pushed us into a money transfer shop and we’ve been sheltering there for over an hour.”
How have leaders reacted?
Barcelona’s mayor, Ada Colau, said the city would hold a minute of silence on Friday at midday.
Spanish Prime Minister Mariano Rajoy said he was heading to Barcelona, adding: “Maximum co-ordination to arrest the attackers, reinforce security and attend to all those affected.”
The United States has offered its assistance to Spain.
Secretary of State Rex Tillerson also said consular assistance was being provided to Americans in the city, and urged US nationals to check in with their families.
“Terrorists around the world should know the United States and our allies are resolved to find you and bring you to justice,” he told reporters.
UK Prime Minister Theresa May said her thoughts were with the victims, adding that “the UK stands with Spain against terror”.
French President Emmanuel Macron said: “We remain united and determined.”
Central boulevard that runs 1.2km (0.75 miles) through the centre of Barcelona
Runs from the city’s Plaça de Catalunya (Catalonia Square) to the Christopher Columbus monument at the seafront.
Popular with tourists because of its market stalls, bars and restaurants
Barcelona city council restricted traffic flow because of heavy pedestrian use of the street
Deadly vehicle attacks
Charlottesville, Virginia, 12 August 2017: James Fields charged with murder after allegedly driving into protesters
Paris, 9 August 2017: A man rammed a BMW into a group of soldiers, injuring six.
London, 3 June 2017: Eight people died when three jihadists drove a van into pedestrians on London Bridge and then stabbed passers-by. A man was killed later in June in a van attack on Muslims in Finsbury Park.
London, 22 March 2017: Four people died when a car rammed into pedestrians on Westminster Bridge, and the driver then stabbed to death a policeman
Stockholm, Sweden, 7 April 2017: Uzbek Rakhmat Akilov killed four people when he drove a lorry into a department store.
Berlin, Germany, 19 December 2016: Tunisian Anis Amri ploughed a truck into a Christmas market at Breitscheidplatz, killing 12 people
Nice, France, 14 July 2016: Tunisian Mohamed Lahouaiej-Bouhlel drove a truck into crowds on the Promenade des Anglais, killing 86 people on Bastille Day
France, December 2014: A van was driven into a Christmas market in Nantes and a car rammed pedestrians in Dijon, leaving more than 20 wounded.