There have been two attacks in Spain’s Catalonia region involving people driving cars at crowds at high-speeds.
Here is what we know so far.
On Thursday afternoon at 16:50 local time (14:50 GMT). a white van smashed into people on Las Ramblas, a famous boulevard in central Barcelona that runs 1.2km (0.75 miles) and was packed with tourists.
The van driver is said to have zig-zagged to try and hit as many people as possible along the pedestrianised area, knocking many to the floor and sending others fleeing for cover in shops and cafes.
He killed 13 people and injured more than 100, and managed to flee the scene.
Spanish police have described it as a terror attack.
What was the second attack?
About eight hours later, an Audi A3 car ploughed into pedestrians in the popular seaside resort town of Cambrils, 110km (68 miles) south-west of Barcelona.
Six civilians were injured, one critically, and a police officer was hurt too.
Five attackers, some of whom were wearing suicide belts, were then shot dead by police. Controlled explosions were then carried out.
Both the Las Ramblas and Cambrils attacks are believed to be linked.
Who has been arrested?
On Thursday, one person from Spain’s north African enclave of Melilla was arrested in Alcanar and a Moroccan was arrested in Ripoll. Both are towns in Catalonia – the same region as Barcelona.
Police say neither of the pair arrested was the driver.
Documents belonging to the Moroccan, 28-year-old Driss Oubakir, were allegedly used to rent the van used in the Las Ramblas attack but local media report he says his papers were stolen and used without his knowledge.
On Friday, police announced another arrest in Ripoll. It remains unclear how many people were involved in the plots.
Weren’t there other incidents too?
Police have shot dead a man who drove into officers at a checkpoint on the outskirts of Barcelona. but the authorities say there is no evidence he is connected to the attack in Las Ramblas.
On Wednesday night, an explosion completely destroyed a house in Alcanar, 200km south of Barcelona, killing one person and wounding seven. Who are the victims?
They come from all over the world, with at least 24 nationalities represented.
People from Ireland, France, Australia, China, Pakistan, Venezuela, Algeria, Peru, Germany, the Netherlands, Greece, Hong Kong, Taiwan and the Philippines are all reported to be among the victims.
Aftermath of Barcelona attack in pictures
Belgium said one of its citizens was killed and France said 26 of its nationals were injured, 11 seriously. The Australian government said at least four citizens were injured.