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.
'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."
The federal government has marshalled its officials in Spain to seek out Australians in the city of Barcelona after a terror attack.
Mossos d'Esquadra Police officers and emergency service workers move an injured after a van crashes into pedestrians in Las Ramblas Source: EFE
"We are concerned that Australians may be involved and the embassy there in Madrid and the consulates that are available are searching for the Australians in Barcelona," cabinet minister Christopher Pyne told Nine.
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.
"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.
"Bourke Street wasn't terrorism and this appears to be, but the result is still the same."
