A manhunt is under way for the terrorist who drove a van into pedestrians in a busy tourist area of Barcelona, leaving at least 13 people dead and more than 100 injured.
Two people have been arrested after a white Fiat van tore through the Las Ramblas district, which was packed with shoppers and holidaymakers on Thursday afternoon.
Police confirmed neither of those in custody was the driver behind the atrocity, which Spain's prime minister Mariano Rajoy described as "jihad terrorism".
Prime Minister Theresa May said she was "sickened" by the incident and the Foreign Office was trying to determine if there were British victims.
A Belgian man was confirmed among the dead, while authorities warned the death toll may rise, with 15 people seriously injured.
The Islamic State terror group claimed responsibility for the attack, the deadliest on Spanish soil since more than 190 people died in the Madrid train bombs in 2004.
The van drove on to the pavement, swerving among pedestrians in a crowded area, and was "clearly a terror attack intended to kill as many people as possible", senior police official Josep Lluis Trapero said.
The two suspects in custody are a Spanish national from Melilla and a Moroccan, he added.
The arrests were made in the northern Catalan town of Ripoll and in Alcanar, the site of an explosion on Wednesday night which killed one person and is potentially linked to the van attack, authorities said.
The whereabouts of the van driver were unclear after police said he made off unarmed.
There were also reports that detectives believe two vans were used, one for the attack and a second as a getaway vehicle.
Hours after Thursday's attack, police shot and killed a man who was in a car that hit two officers at a traffic blockade on the outskirts of Barcelona. However, Mr Trapero said it was not linked to the van attack.
Following the deadly incident in Las Ramblas, police were searching for a man named Driss Oukabir after the passport of a Spanish citizen, of Moroccan origin, was found at the scene.
But El Pais newspaper said Oukabir had denied being involved in the attack and told police that his documents had been stolen.
Witnesses to the attack described how the van hurtled through crowds without slowing down, sending scores of people scattering and many taking shelter in nearby shops.
Lawyer and University of Glasgow rector Aamer Anwar told the Press Association: "I could see a woman screaming with her kids. People started running and jumping into shops.
"I ran for about 50 or 100 metres and stopped to see what was happening.
"The police were very quickly on the scene and getting people to move back."
Will Ako, 26, from London, was eating in a restaurant called Taller De Tapas with his family when they heard a disturbance.
He told the Press Association: "I then saw people running to the east of the restaurant and I saw a couple of people, about 30 yards down the road, crouching around what looked like someone on the ground and they were calling for assistance.
"Within a couple of minutes, police started to arrive and they were running west of the restaurant and some were telling us to stay inside.
"Soon after, loads of cars, bikes and ambulances arrived and then the armed police."
Mrs May said the UK "stands with Spain against the evil of terrorism" in the wake of recent attacks in Manchester and London.
"I am sickened by the senseless loss of life in Barcelona today," she said.
"The Foreign Office is working to establish if any British nationals were involved in this appalling incident and we are in close contact with the authorities in Spain, who have our full support."
The Government of Catalonia said regional president Carles Puigdemont had declared three days of mourning.
Shortly after midnight there were reports of police activity in the coastal town of Cambrils, around 68 miles west of Barcelona.
The 112 Catalonia emergency service tweeted: "IF YOU'RE NOW IN #Cambrils avoid going out. Stay home, stay safe. Police operation ongoing."
The Catalan police force, Mossos, tweeted: "Police operation in #Cambrils for possible terrorist attack. If you are now in #Cambrils avoid going out. Stay home, stay safe."
There were reports on social media of the sound of gunfire ringing out in the popular Costa Dorada tourist spot.
Video posted on social media appeared to show bystanders close to a number of emergency vehicles taking cover as shots pierced the air.
The footage appeared to have been taken near Av de la Diputacio, a beachfront road lines with hotels, shops and restaurant.