Barcelona was struggling to its feet on Friday after a day of terror at the height of the holiday season that left four Australians injured, two seriously, and another missing.

The missing person was reportedly a seven-year-old boy, whose mother was one of those in hospital, seriously injured in the attack.

The London Telegraph identified the missing boy as seven year-old Julian Cadman, whose mother Jom was being treated in hospital after being seriously injured in the van attack.

A relative posted on Facebook, "Julian is 7 years old and was out with Jom when they were separated, due to the recent terrorist activity. Please share, especially if you have family or friends in Barcelona. Thank you."

At least 13 people were killed and more than 100 injured when a van ploughed into tourists on the Spanish city's central Las Ramblas boulevard on Thursday afternoon.

It was Spain's worst terror attack in more than a decade, and the sixth time in recent years that terrorists have used vehicles as weapons in Europe. Many of these times they have targeted areas popular with tourists: a Berlin Christmas market, Westminster Bridge in London, and the seafront in Nice. The Islamic State claimed responsibility for the Barcelona atrocity.

Eight hours later, police shot dead five suspected terrorists in the seaside resort town of Cambrils, 110 kilometres south-west of Barcelona, after they drove a car into pedestrians.

A woman injured in the second attack, which left six others injured, died from her wounds, police say. Her death brings the toll from both attacks to 14.

Police said the men were wearing explosive belts and a Catalan government minister said they were linked to the Barcelona attack.

The belts were later found to have been fake.

In the Barcelona attack, one Melbourne man leaped out of the way of the van, which was travelling at high speed and missed him by centimetres, then helped with first aid for those left in the van's wake.

Several other Australians had stories of narrow escapes.

Eyewitnesses spoke of the screams of tourists on the packed thoroughfare, of the sick thudding sound as the van smacked into the crowd, and of the bodies of men, women and children flying into the air then lying on the street after the attack.

They said the van had mounted the crowded pedestrian path that ran along the middle of Las Ramblas and drove along it for 500m, weaving back and forth as it sought out victims.

"All of a sudden, the police just shouted at everyone, telling them just to run. There was a really loud kind of crashing noise. I didn't stop to look back," Ethan Spieby, a witness caught up in the commotion, told the BBC.

Tom Markwell, another American, told the BBC he saw a white van "going entirely too fast. It looked to me as if he was going left to right, hitting people with the little stand ... All of a sudden, people were just screaming and running."

Police arrived quickly on the scene, but on Friday morning, Spanish time, the driver of the van was still at large. One eyewitness told the Telegraph that he saw two people get out of the van after it stopped. Others saw just one man, who fled on foot.

There were at least 34 nationalities among the dead and injured. One of those killed was a three year-old child, Spanish media reported. Authorities warned that the number of victims could rise as 15 people were in critical condition in hospital.

The dead included three Germans and one Belgian tourist. Of the wounded, 26 were from France - of whom 11 were seriously injured.

Foreign minister Julie Bishop said eight Australians had been affected. Two women from NSW were in serious but stable condition at hospital, two men from Victoria were treated at hospital for light injuries then discharged, three people had asked for consular support, and one person was unaccounted for.

Ms Bishop said she was concerned for the missing person, and Australia was working with Spanish authorities to try to find them.

Prime Minister Malcolm Turnbull said this was a "global battle against terrorism", and that Australia had been working towards a policy on protecting crowded places in Australia, which would be released shortly.

Ms Bishop condemned the "brutal and cowardly attacks, clearly designed to harm and affect tourists".

The tree-lined La Rambla, known as Las Ramblas, is the main tourist thoroughfare leading from the harbour into the heart of the city.

On Friday morning, after a night in which much of the city centre was cordoned off by a heavy police presence, the street re-opened and locals began setting up their stalls again, as the city tried to return to normality.

The New York Times reported that police believed the attack had originally been intended to involve explosives.

Spanish police were investigating a possible connection between the van attack and a gas explosion the previous night at a house in Alcanar, south of Barcelona, in which one person died.

Police said the residents had been preparing an explosive device.

One Spanish citizen had been arrested in Alcanar, and a second man, a Moroccan named Driss Oukabar, turned himself into police after his identity documents were used to hire the van used in the Las Ramblas attack.

Spanish media reported that the man's brother may have stolen his identity documents.

A third man was arrested in the town of Ripoll, while Catalan police later said they had made a fourth arrest in relation to the attacks, but gave no details.

The New York Times reported that three vans had been rented under Oukabar's name, and the other two had still not been located.

"Part of the plan was they tried to rent a larger truck, but they didn't have the right permit" so they ended up getting smaller vans, a counter terrorism expert told the Times.

The Spanish prime minister announced three days of national morning after what he called a "jihadist attack".