A van has ploughed into crowds in Barcelona's famous Las Ramblas tourist area, the BBC reported a short while ago.
It said people were killed and injured as the vehicle sped along the pedestrianised area, sending many fleeing for cover in shops and cafes.
Witnesses had said the van had deliberately targeted people before coming to a stop.
Quoting the El Pais newspaper, the BBC said the driver of the vehicle had fled on foot after mowing down dozens of people.
Police are treating the incident as a terror attack.
Spanish media reports say one or two armed men barricaded themselves inside a bar in the area after the attack.
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 has 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.