LAS RAMBLAS REOPENS AS FOURTH SUSPECT IS ARRESTED IN BARCELONA TERROR ATTACKS

A fourth person has been arrested over a series of suspected terror attacks that killed 14 people and injured 130 in Spain, authorities say.

Key points:

°Five assailants shot by police, four people arrested, police still searching for another four

°Police say media reports the driver of the van was among those shot can't be confirmed

°Four Australians have been injured, one seven-year-old boy is missing

°Police shot dead five would-be attackers after confronting them in Cambrils, a town south of Barcelona where hours earlier a suspected attacker drove a van into crowds.

°Authorities were continuing an urgent manhunt for the driver of the van that ploughed into crowds on the popular Las Ramblas tourist strip.

Of the four people already arrested, three were from Morocco and one was from Spain, police said.

They were aged between 21 and 34, and none had a history of terrorism-related activities.

Authorities have issued arrest warrants for four further suspects in connection with the two attacks, a judicial source said, declining to give their names.

What we know about the attack

A quick look at the details of the Barcelona attack, including the death toll, number of injured, those responsible and descriptions of the scene.

Josep Lluis Trapero, police chief in Spain's north-eastern region of Catalonia, said he could not confirm media reports that the driver of the van that ploughed into the crowds was one of those shot in Cambrils.

"It is still a possibility but, unlike four hours ago, it is losing weight," he told regional TV.

Mr Trapero said the investigation was focusing on a house in Alcanar, south-west of Barcelona, which was razed by an explosion shortly before midnight on Wednesday (local time).

Police believe the house was being used to plan one or several large-scale attacks in Barcelona, possibly using a large number of butane gas canisters stored there.

However, the apparently accidental explosion at the house forced the conspirators to scale down their plans and to hurriedly carry out more "rudimentary" attacks, Mr Trapero said.

Authorities in Vic, a small town outside Barcelona, said a van had been found there in connection with the attack. Spanish media had earlier reported a second van had been hired as a getaway vehicle.

A French police source told Reuters authorities there were looking for the driver of a white Renault Kangoo van that may have been used by people involved in the Barcelona attack.

Las Ramblas was reopened the morning after the attack , though a police presence and security tape was still visible in some places.

Thirteen people were killed in the attack at Las Ramblas, and a 14th died from injuries suffered from a related attack at Cambrils.

Four Australians are among the 130 people injured , and authorities have warned the death toll could rise.

A seven-year-old Australian boy was missing and his mother was in a serious condition in hospital.

The Islamic State group claimed responsibility for the attack on its Amaq website, and it was carried out "in response to calls for targeting coalition states" — a reference to a US-led coalition against the Sunni militant group.

Spain has several hundred soldiers in Iraq providing training to local forces in the fight against Islamic State, but they are not involved in ground operations.

If the involvement of Islamist militants is confirmed, it would be the latest in a string of attacks in the past 13 months in which they have used vehicles to bring carnage to the streets of European cities.

A minute's silence in Barcelona turned political, when some mourners raised Spanish and Catalan flags.

They were quickly rebuked them for trying to politicise the solemn event at Las Ramblas, with the crowd urging them to "get rid of the flags".

Polls show the region is split ahead of a planned referendum on whether Catalonia should become independent from Spain, which the country's central Government considers would be illegal to hold, in October.

"We're here for the victims and to protest what happened. This is not about anyone's politics," Anna Esquerdo, a lifelong Barcelona resident, said.

Everything changed in a 'split second'

Witnesses to the van attack said the white vehicle had zigzagged at high speed down Las Ramblas, ramming pedestrians and cyclists, sending some hurtling through the air and leaving bodies strewn in its wake.

Mobile phone footage showed several bodies strewn along the Ramblas, some motionless.

Paramedics and bystanders bent over them, treating them and trying to comfort those still conscious.

The injured and dead were of 34 nationalities, emergency services said.

Foreign Minister Julie Bishop said eight Australians were caught up in the attack: four were injured and treated in hospital, and one person was unaccounted for.

Sydney woman Lauren Grundeman, who was walking along Las Ramblas just before the attack, said there was a "great vibe" in the city before terror suddenly struck.

"So many people were out — lots of families, lots of little children — it was great, the weather was beautiful," she said.

"About five we decided to go back to the hotel and we went down a side street, then all of a sudden we heard lots of people screaming, crying and yelling, and I was like 'what's going on here?'"

Australian woman Julia Monaco said there was "mild confusion" initially before the horror of the attack was revealed.

"It was a split second and everything changed and suddenly, everybody in that square was just running," she told ABC News Breakfast.

"Whatever they had seen or heard, had terrified them truly, and they just started running."

Attack comes at height of Barcelona tourist season

Spanish Prime Minister Mariano Rajoy announced three days of official mourning for what he called a "jihadist attack".

The Spanish royal household said on Twitter: "They are murderers, nothing more than criminals who are not going to terrorise us. All of Spain is Barcelona."

Regional head Carles Puigdemont said people had been flocking to hospitals in Barcelona to give blood.

The attack is the deadliest in Spain since March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.

The incident took place at the height of the tourist season in Barcelona, which is one of Europe's top travel destinations with at least 11 million visitors a year.

French President Emmanuel Macron, whose nation has suffered some of Europe's deadliest militant attacks in recent years, tweeted: "All my thoughts and France's solidarity to the victims of the tragic attack in Barcelona."

Before the Las Ramblas attack, Government data showed that police had arrested 11 suspected jihadists in the Barcelona area so far this year, more than anywhere else in Spain.