Eyewitnesses of the deadly car rampage on Barcelona's popular Las Ramblas have told of hearing screams and seeing bodies, as a car ploughed into pedestrians just after 5pm local time.

Gil Van Der Venne, an Australian man holidaying in Spain, has told Melbourne radio station 3AW of how he witnessed the Barcelona attack from his hotel balcony.

More than 13 people were killed and 100 injured when a car rammed through a crowd of pedestrians in the area's tourist district.

Mr Van Der Venne said had got back to his hotel after a day's sightseeing and saw the car careening through the Las Ramblas area from his hotel's rooftop bar.

"[I] heard the screaming, the mayhem. [I] looked over the top and there were at least five bodies that I saw, three of them now in body bags," he said.

"It was very, very scary."

He said the hotel where he was staying, Bagues, was still in lockdown following the attack.

Pablo Morante, lives near Barcelona and was leaving work and on the bus home when the attack happened.

"I was on the bus and when I left it I saw the [white] van coming down the street and lots of people running everywhere," he told Fairfax Media.

"I saw bodies on the ground.

"The moment I saw it, it was very fast and intentional for sure."

He said he knew it was a terrorist attack immediately.

'We were literally running for our lives'

Australian woman Julia Monaco was shopping in the Las Ramblas area with a friend when the attack unfolded outside. It is the third terror attack the young traveller has been caught up in the past three months.

Ms Monaco was in lockdown in the London Tube during the London Bridge attack in June and she was in Notre Dame days later when another terror attack took place.

"All of a sudden we were locked in the shop and confusion started ... nobody was really sure what was happening," she told Melbourne radio station 3AW.

"The music was still playing in the shop ... suddenly we could see that the police presence was rapidly increasing.

"Then we started seeing people looking incredibly distressed.

"The next second we knew, we were literally running for our lives to the very back of the store where we were told to get on the ground, away from the windows and just lay as flat as possible.

"We hid behind a clothing display that was about knee-high.

"There was considerable panic. There were two children who were screaming.

"Thankfully, I can say I'm OK."

Despite her experience, Ms Monaco said she won't be taking her parents' advice and cutting her European adventure short.

"I feel a bit rattled ... but I don't feel like I want to go home ... I don't want to let 'them' win. I'll finish what I started.

"[I] still want to travel the world, maybe there's something wrong me but I'll keep going."

'I heard screams'

Tom Gwella told the BBC's Radio 4 Today program the van was going "full pelt" down the street.

"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," he said.

"It wasn't slowing down at all. It was just going straight through the middle of the crowds in the middle of the Ramblas."

Footage seen by Fairfax Media of the immediate aftermath of the attack shows numerous people, many of them women, covered in blood lying on the ground. Some are motionless, others are partially clothed.

Police and other emergency services personnel are seen treating some of the wounded.

Rhys Richards told the BBC he was ten feet from the attack.

"I saw one person lying on the ground receiving CPR and another person lying on the ground motionless."

Aamer Anwar, a lawyer and a rector at Glasgow University, was on Ramblas at the time of the attack.

He said on Twitter he had been walking at the spot where the car drove into people just 10 seconds prior. He said the "cowards" had attacked a "sea of humanity".

"I'm safe, shocked and lucky," he wrote.

"10 seconds away, had walked on because it was so busy."

'Can't wait to be home'

Sharna Beckman, a young woman from the NSW central coast, is cutting her holiday short and flying home to Australia in the wake of the attack.

She was in a taxi in the Las Ramblas area when she was confronted by police with "massive guns".

"We were in a taxi and then all of a sudden there was all this traffic and then had these cops with massive guns telling us to move and then we got out of the taxi not knowing what was going on," she told Fairfax Media.

She made it back to her hotel, where she spent the next seven hours in lockdown, before deciding to head to the airport.

"We were told that it was best to stay inside, but we just wanted to get out of Barclona so we got a taxi and left," she said.

"Can't wait to be home and feel safe."

- with Madeleine Heffernan