At least one person died and 32 were injured after a van ploughed into crowds in Barcelona's famous Las Ramblas tourist area, police say.
The vehicle sped along the pedestrianised area, mowing down people and sending others fleeing for cover in shops and cafes.
Witnesses said the van had deliberately targeted people before coming to a stop.
Police are treating the incident as a terrorist attack.
Media reports said the driver of the vehicle had fled on foot.
Tom Markwell from New Orleans, who was just arriving in a taxi in Las Ramblas, said: "I heard a crowd screaming. It sounded like they were screaming for a movie star.
"I saw the van. It had already been busted on the front. It was weaving left and right, trying to hit people as fast as possible. There were people lying on the ground."
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."
Vehicles have been used to ram into crowds in a series of attacks across Europe since July last year.
Some of the early reports suggested Catalonian nationalists what with some of them being averse to tourism
Spanish police say they have killed five suspected terrorists in the town of Cambrils in a second vehicle attack, after an earlier one in Barcelona.
The men, wearing explosive belts, were linked to the first attack, police say.
Thirteen people died and dozens were hurt when a van hit crowds on Las Ramblas, Barcelona on Thursday. The driver fled and is still at large.
Spain's PM Mariano Rajoy described it as a "jihadist attack", which so-called Islamic State said it had carried out.
Seven people, including a police officer, were wounded when a car was driven into them early on Friday, Catalan emergency services said. One person is in a critical condition.
The attackers' vehicle overturned and when the men got out they were quickly fired upon by police, media say. One was reportedly brandishing a knife.
A series of controlled explosions was then carried out.
Police say the situation in Cambrils - a popular seaside resort 110km (68 miles) south-west of Barcelona - is now under control.