Thirteen people have died and around 100 have been injured after a van mowed into crowds in Barcelona's Las Ramblas tourist area yesterday.
The van sped down a pedestrianised area hitting people while others jumped into nearby shops and eateries for protection, with the driver reported by local media to have fled on foot.
Catalan government officials have confirmed the number of casualties and say two men have been arrested; a Moroccan in Ripoll and a Spanish national in Alcanar. An earlier explosion in Alcanar, which killed one person, has been linked to the Las Ramblas attack. Another van connected to the terror was found in the town of Vic.
Police last night released a picture of a suspect called Driss Oukabir that they want to question; it is alleged he rented the van.
Police were involved in a counter-terrorist operation in the early hours of this morning in the town of Cambrils near Tarragona.
The so-called Islamic State group last night told the group's Amaq news agency it carried out the attack.
The Barcelona attack is the latest in a string to blight European cities. Recent acts of terror include the 2015 Bataclan attack in Paris and 2016 assaults on the streets of Nice, Berlin and Stockholm.
Meanwhile, this year alone terrorists have targeted civilians in Westminster, London Bridge and Manchester, killing dozens, as well as a claimed attack in Finsbury Park.
Prime Minister Theresa May said the UK "stands with Spain against terror" following the van massacre.
Spanish Prime Minister Mariano Rajoy tweeted: "The terrorists will never defeat a united people who love freedom in the face of barbarism.
"All of Spain is with the victims and families."
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."
Mayor of London Sadiq Khan said on Twitter that his thoughts were with the victims of "this barbaric terrorist attack".
London stands with Barcelona against the evil of terrorism.
Meanwhile, US President Donald Trump said said he will do "whatever is necessary to help".
Steven Turner, who works in the area, told the BBC: "People in my office saw a van ramming into people on Las Ramblas.
"There are lots of ambulances and armed police with assault rifles around now."
Aamer Anwar said he was walking down Las Ramblas, which was "jam-packed" with tourists.
"All of a sudden, I just sort of heard a crashing noise and the whole street just started to run, screaming. I saw a woman right next to me screaming for her kids," he told Sky News.
"Police were very, very quickly there, police officers with guns, batons, everywhere. Then the whole street started getting pushed back.
"Police officers who got there just started screaming at people to move back, move back."
Local metro and train stations were closed and the Las Ramblas area was still on police lockdown late last night.