Barcelona's most famous street was packed with tourists when a van drove into the crowds on a sunny Thursday afternoon, leaving scenes of carnage and panic.
"There were bodies on the ground with people crowding round them. People were crying," Xavi Perez, who sells sports magazines just 100 metres away from the attack, told AFP.
Three Australians have been injured in the Barcelona terror attack.
One Australian woman is in a serious but stable condition, Foreign Minister Julie Bishop said.
She said two young Australian men have returned to their hotel and will be seeking treatment for their injuries.
Meanwhile, an Australian tourist has told of the horror that unfolded moments after a van ploughed into crowds, killing at least 12 people in a terrorist attack.
Police officers evacuate people from a shop at Las Ramblas as they take security measures after a van ploughs through a main street in Barcelona. Source: Getty
'Absolute terror'
Susan McLean was about 100m from the scene as the van zigzagged down one of the city's busiest avenues, mowing down pedestrians and leaving bodies strewn across the ground.
"All of a sudden there was this tidal wave of people running from both Placa de Catalunya and Las Ramblas towards us screaming, crying and with absolute terror etched on their faces," she told Nine Network on Friday.
"Initially we couldn't hear anything other than all these people running towards us. Several of them were calling 'gun, gun', so first of all we thought someone had been shot.
"Then they just kept sort of - it was all in Spanish. It was very difficult to understand but they were sort of pushing us into shops."
Among the foreigners caught up on Las Ramblas was Aamar Anwar, a renowned Scottish human rights lawyer in Barcelona for a conference and had been walking down the boulevard when the terror unfolded.
Injured people react after a van crashed into pedestrians in Las Ramblas, downtown Barcelona, Spain, 17 August 2017. Source: AAP
"All of a sudden I heard a crashing noise and the whole street just started to run screaming," he told Sky News, describing a scene of thousands of people struck by chaos and panic.
"I saw a woman next to me screaming for her kids.
"Literally within 30 seconds, police vans, ambulances, police officers with guns were piling out, and we were sectioned off and then being pushed rapidly back," he said.
Another witness said he saw a man fleeing.
"I saw a man run down the Ramblas, with police chasing him and he appeared to drop a black metal object. It looked like a pistol," said the witness who only gave his first name, Sergio.
"All of a sudden, scores of people ran towards us -- hysterical, children hysterical."
As people ran for their lives they were replaced by armed police officers who sealed off the scene.
"Van upon van of police officers" then arrived, Anwar said. "They have quite clearly unfortunately had to plan for something like this."
The federal government has marshalled its officials in Spain to seek out Australians in the city of Barcelona after a terror attack.
The government wants any Australians in the city to let people know they are safe - either through Facebook, by calling relatives or contacting the consulate in Spain.
Australia stands resolute with Spain: PM Turnbull
Prime Minister Malcolm Turnbull said the love and prayers of Australians are with the victims of the attack.
"Resolute with Spain in the fight to defeat terrorism we condemn the terrorist attack in Barcelona," he tweeted.
Ms McLean, who was separated from her husband in the panic, said the scene reminded her of Melbourne's January 20 Bourke Street Mall killings, when a man in a car mowed down pedestrians. Six died and up to 30 were injured.
"That was quite scary because I wasn't sure where he was but the shopkeepers were great. They pulled everyone inside and put the shutters down.
'My first reaction was the Bourke Street massacre'
"My first reaction was the Bourke Street massacre, because that is what it reminded me of, the vision of people fleeing in just such terror."
Australian Gil Van Der Venne told Melbourne Radio 3AW he saw the attack unfold from his hotel balcony.
Policemen check the area after towing away the van which ploughed into the crowd in Barcelona. Source: Getty
"We're looking over the balcony ... and there it is all happening in front of you," he told Melbourne Radio 3AW.
"You hear this screaming, the mayhem.
"There was at least five bodies that I saw, at least three of them are still there now with body bags, so you'd imagine they've passed.
"The van, I believe, had probably travelled about three-quarters down Las Ramblas."
Melbourne holiday-maker Julia Monaco told Nine police on the scene suddenly ordered her and her family inside a shop just across from Las Ramblas as the attack happened.
"In a split second it all kind of changed and everyone just started running and panicking and running for their lives and crying and screaming and we were forced back into the store, told to get away from the windows and to get low on the ground.
"We were huddled at the very back of the store lying flat on if floor and were in that position for about 20 minutes with no real understanding of what was happening other than it was incredibly serious."
'Horrific scene'
A Melbourne firefighter says the carnage that has unfolded in Barcelona reminds him of the Bourke Street rampage, in which he was a first responder.
"I heard a large rumbling sound to begin with, a shot.. and some time after that screams and the horrific scene down there," said MFB Commander Graeme O'Sullivan, who witnessed the attack.
He said the scene reminded him of Melbourne's January 20 Bourke Street Mall, when a man in a car mowed down pedestrians, killing six people.
"It's very similar [to Bourke Street]," he told 3AW.
Spanish Prime Minister Mariano Rajoy during the meeting with senior officials of the State Security Forces after the Barcelona attack. Source: AAP
"Bourke Street wasn't terrorism and this appears to be, but the result is still the same."
Tom Gueller, who lives on an adjoining road, fled the scene when he saw the van hurtling through the crowds.
"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," he told BBC radio.
"I ran away, I mean I live near, I had to run back about 50 metres or so and go up to my flat and obviously see what's happening on the road from my balcony."
Steve Garrett was in a nearby market and sheltered in a bakery with several others.
"Coming from England it was reminding me a great deal of what happened in London, so we were very concerned about what might happen next."
Armed police then appeared.
"They seemed to sweep through the market area. They seemed to be looking for someone. They were going very carefully, very cautiously, stall to stall," he said.
