Citizens from at least 24 countries have been injured or killed in the van attack in the Las Ramblas area of Barcelona and the coastal town of Cambrils, according to Catalan authorities.
By Friday the death toll stood at 13 bystanders killed and more than 100 injured.
Authorities said so far, the youngest victim of the attacks is thought to be a three-year-old girl.
She died shortly after she was taken to hospital.
A six-year-old girl of unknown nationality has also been taken to hospital with a cerebral haemorrhage, an official at Vall d’Hebron university hospital told the New York Times.
France has confirmed that there are 26 of its citizens injured with 11 in a serious condition.
The Spanish newspaper La Vanguardia, quoting police, said three Germans were also among the fatalities.
Belgium’s Foreign Affairs Minister, Didier Reynders, confirmed on Twitter that one of its citizens was killed in the attack, and also said the Belgian embassy was in touch with hospitals in the area regarding other possible victims.
The Hague said three Dutch were injured, and a Greek diplomat reported three nationals had been wounded – a woman and her two children.
China also confirmed a citizen of Hong Kong had minor injuries, while the U.S. state department said it knew of one American with minor injuries.
In Australia, Foreign Minister, Julie Bishop, said four Australians were hurt: two women in a serious but stable condition, and two men, who were “directly affected” and had retreated to their hotel to seek medical attention in the morning.
One Australian is still missing, the minister said.
Throughout the long, chaotic night there were urgent appeals on social media for English, Italian and French translators to make their way to hospitals and clinics to assist staff attending the dozens of non-Spanish speaking victims.
Ireland’s Department of Foreign Affairs confirmed it is helping an Irish family of four caught up in the attack.
A father of two, originally from the Philippines but who lives in Ireland was on holiday with his wife and two children, when he was hit on the side.
He may need an operation but his injuries are not life threatening, Dr Emmanuel Fernandez, Consul General of the Embassy of the Philippines in Madrid, told the Irish broadcaster RTE.
Overnight, many countries set up emergency helplines for their citizens caught up in the violence, and sent out warnings for those trapped in the affected area to remain inside while counter-terrorism efforts were underway.
Spanish police meanwhile swept through the narrow alleys branching off Las Ramblas, past deserted outdoor cafes, where half-eaten plates of tapas lay abandoned by diners who fled for their lives.
As Spain wakes, foreign embassies are still scrambling to establish, who has been affected. Many tourists are not believed to have registered their travel plans and movements.
Spanish authorities are yet to publish the names of the dead and injured.
Many parts of central Barcelona remained sealed off overnight, with guests told to stay in their homes or hotels, and the metro and most transport routes shut down.
Families and friends have been split up, with police evacuating people to different parts of the city, and mobile phone networks overwhelmed.
Catalan authorities urged those affected to stay off the phone networks and use social media to inform their loved ones they were OK.
Facebook activated its safety check device for the attack, as it has done for the terror attacks in Manchester and London.
In Paris, the Eiffel tower turned off its lights as a sign of respect to the innocent victims caught up in the violence, and in New York the world trade centre spire was lit with the colours of the Spanish flag.
Graphic videos of the attack were widely posted on social media, with some chastising people who posted the footage for filming rather than helping the injured.
So far, the British foreign office has received no confirmed reports of British citizens injured or killed, but Chris Pawley, 30, a survivor of the Manchester bombing, was visiting Spain with his partner and was in the area when the attack occurred.
He told the Manchester Evening News he couldn’t believe he had been caught up in a second terrorism incident in fewer than six months.
He had just left the Ariana Grande in May when the bomb detonated.
Pawley said of Thursday’s attack: “There was police everywhere and ambulances, the shops started putting the shutters down.”
“We have just come back to the hotel, as we were caught up in the arena attack – can’t believe it.”
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