- Video report by ITV News Reporter Martha Fairlie
At least 13 people are dead and 100 injured after a van mounted the pavement in the popular Las Ramblas tourist area of Barcelona.
Officials confirmed it was a "terrorist attack", and said their "attack protocol has been activated".
Police said three suspects had now been arrested in connection with the attack.
Earlier they said they were still hunting the driver of the vehicle.
So-called Islamic State has claimed responsibility for the attack.
Police in Barcelona cordoned off the area immediately after the attack and asked people to stay away from the scene.
Images posted on social media shortly after the crash appeared to show people lying on the street being treated for injuries.
A photograph shown by public broadcaster RTVE showed at least three people on the ground apparently being helped by police.
People at the scene said they saw people running away and screaming after the incident happened as the driver of the van was reported to have fled on foot.
One eyewitness, Max Gayler, described the scenes of chaos to ITV News.
Another eyewitness Aamer Anwar, who was walking down Las Ramblas at the time of the crash, said the area was "jam-packed" with tourists when the van mounted the pavement.
Mr Anwar described the scene as "chaos" and said: "I turned around and people were screaming - I could see a woman screaming with her kids - people started running and jumping into shops.
"I ran for about 50 or 100 metres and stopped to see what was happening. The police were very quickly on the scene and getting people to move back.
"I could see chaos right at the top area and I spoke to a shopkeeper who had run down and was screaming.
"He was Bengali so I spoke to him in Urdu and he said a van had driven into a crowd and he thought there were five to six people very seriously injured."
He also tweeted a video of the scene along with a message which read: "Can't believe this I'm on Ramblas, heard screaming & whole street ran- a car believed drove in2crowd- had walked down 10secs earlier."
In a different video shared on Twitter by social media user Arturo Fernandez Rui police can be seen surrounding the main stretch of Las Ramblas in the aftermath of the attack.
In a second video, showing more police rushing to the scene, Mr Rui wrote: "The horror comes to our city."
Simon Manley, the British ambassador to Spain, has confirmed the Foreign Office is "in contact with the authorities and seeking further information following the incident".
While a Foreign Office spokesman said they were "in contact with the Spanish authorities and seeking more information following an incident in Barcelona" and "anyone in the area should follow the advice of the emergency services."
Barcelona's Mayor has announced that a minute's silence will be held on Friday in the city's main square 'to show that we are not scared' after van attack.
World leaders react to attack
Catalonia's regional president Carles Puigdemont described the attack as "shocking" and pledged "all of us will join together to defeat those who use violence. Barcelona, Catalonia is a city of peace"
Prime Minister Theresa May said she was "sickened by the senseless loss of life in Barcelona" and that "Britain stands with Spain against the evil of terrorism."
US President Donald Trump condemned the attack in Barcelona and posted a message on social media saying the US "will do whatever is necessary to help. Be tough & strong, we love you!"
Belgian Prime Minister Charles Michel also sent a message of support to those caught up in the attack in Spain, tweeting "no barbaric act will undermine the power and resilience of our ally."