Witnesses have described hearing people scream as they ran from the scene in Barcelona where a van crashed into crowds of people.
At least 13 people died and 50 were injured when the van crashed on a pavement in the Las Ramblas area,a broad, long street of stalls and shops, on Thursday afternoon.
Police are treating it as a terror attack.
Briton Ethan Spibey, who was nearby when it happened, described having to flee.
He was later locked in a church near Las Ramblas and said police had told him a suspect was on foot.
The church was providing food and water to "visibly shaken" people it was sheltering.
Spibey told Sky News: "I was just walking towards Las Ramblas with my boyfriend and there seemed to be some police there already.
"All of a sudden there was just real kind of chaos. People just started running and screaming.
"There was loud bangs and people just started running into shops.
"There was a mini stampede where we were down one of the alleyways.
"So we've just now been locked into a church.
A police source has said the driver of the van fled the vehicle on foot after crashing into a group of people.
Tom Gueller told the BBC: "I heard screams and a bit of a crash and then I just saw the crowd parting and this van going full pelt down the middle of the Ramblas and I immediately knew that it was a terrorist attack or something like that."
Australian Susan McLean she she saw "the panic and people running" and had now taken shelter in a shop around the corner.
Reports of the number killed varied massively as the developing picture emerged. Local radio reported 13 people dead at one point.
A woman who witnessed what happened and sheltered in a shop nearby, told Spanish TV: "I see the police and ambulances pass by.
"I am barricaded inside. A lot of people have come to hide in the store. We have a family with little girls in here."
Another witness said: "A lot of screams have been heard and I went out to the balcony.
"There are a lot of people lying on the ground, a lot of blood. Some of them looked like they were dead."
Police urged people not to share pictures and video of the victims. One video showed people on the ground in the pedestrianised middle of the street.
Footage showed the police presence at the scene and what was left of the van.