(CNN) Witnesses of Barcelona's van ramming attack on Thursday described victims "flying into the air" and a "tidal wave" of people running, as a van plowed into a crowd on one of the city's busiest streets.

Spanish police are treating the incident that killed at least 13 people on the Las Ramblas avenue as a terror attack.

Ali Shirazinia said the street was, as usual, teeming with tourists, street merchants and performers on what was a beautiful summer's day.

#Barcelona Eyewitness Ali Shirazinia describes horrific moment of attack: "I saw people flying into the air" https://t.co/gmRe2JBTEF

He was riding his bike down the street when he suddenly heard screams.

"I looked over to my left and I saw all of the people along the promenade kind of split into two -- some going right, the rest coming really towards me, screaming and running as fast as they could," he told CNN.

That's when he saw the white van mowing down the crowd.

People fleeing the scene in Barcelona on Thursday after the attack.

"I saw people flying into the air and everybody was kind of running into the shops on either side of the Ramblas, and a lot of people were in shock."

'Screaming in terror'

Tourist Susan McClean told CNN of the moment she saw a large crowd of people running towards her.

JUST WATCHED Witness: People were screaming in terror Replay More Videos ... MUST WATCH Witness: People were screaming in terror 00:40

"All of a sudden there was this tidal wave of people running towards us, and they were hysterical," she said.

"Children were screaming, there was a lot of distress."

She ducked into a nearby shop and the shutters were pulled down while police sped towards the scene. She couldn't understand what people were saying in Spanish, but she knew it was bad.

"You could see the fear and the distress in these people, and the fact that they were screaming in terror. Regardless of what might have happened we knew we had to get ourselves out of there," she told CNN.

Hiding in a bathroom

Ella Bartlett had just sat down with some friends at a restaurant in the nearby Plaza Real for a meal. They were looking at a menu when there was a sudden commotion.

"Once we saw that everyone was running, we just started running too. We didn't even know what we were running from," she said

Some people hiding in a restaurant called Bartlett and her friends to join them, and they hid in a bathroom in the back with two other people, one of them a girl in tears.

"There were some people in the restaurant lobby looking out and all we could see from the windows was just people running around in the Plaza. It was really scary."

Shirazinia, who lives in Barcelona, said that he and some friends had been worried an attack was "always around the corner."

"I know from speaking to a lot of of my friends from Barcelona, whenever tragedy has struck in other cities, around the world, in Europe especially, everybody felt that the Ramblas would be next. I don't think a lot of people are that surprised that it happened here," he said.

Photos: Terror attacks in Spain A makeshift memorial pays tribute to those who were killed in a terror attack in Barcelona, Spain, on Thursday, August 17. A van rammed into a crowd of people near the popular tourist area of Las Ramblas. Early the next morning, a group of five attackers drove into pedestrians in the Spanish town of Cambrils, about 75 miles south of Barcelona. Hide Caption 1 of 19 Photos: Terror attacks in Spain Two women cry at the street memorial in Barcelona on Friday, August 18. Hide Caption 2 of 19 Photos: Terror attacks in Spain A man embraces a police officer in Cambrils on August 18. Earlier in the day, officers had engaged in a shootout with five attackers who drove a car into several pedestrians. All five were shot dead by police, four of them by one officer, police said. Hide Caption 3 of 19 Photos: Terror attacks in Spain Pigeons fly over a Barcelona crowd that gathered for a minute of silence on August 18. Hide Caption 4 of 19 Photos: Terror attacks in Spain Spain's King Felipe VI joins other officials in observing a minute of silence in Barcelona's Plaça de Catalunya. Hide Caption 5 of 19 Photos: Terror attacks in Spain Police officers investigate a car in Cambrils. Hide Caption 6 of 19 Photos: Terror attacks in Spain The van that plowed into the crowd in Barcelona is towed away from Las Ramblas on August 18. Hide Caption 7 of 19 Photos: Terror attacks in Spain Police officers check the area after towing away the van. Hide Caption 8 of 19 Photos: Terror attacks in Spain Police accompany clients of a store outside a cordoned off-area of Barcelona. Las Ramblas is especially crowded in the summer, the height of tourist season. The promenade passes by kiosks, flower sellers, cafes and bars. Hide Caption 9 of 19 Photos: Terror attacks in Spain Police officers work at the scene in Cambrils. Hide Caption 10 of 19 Photos: Terror attacks in Spain Police check people's identities as they clear Las Ramblas on August 17. Hide Caption 11 of 19 Photos: Terror attacks in Spain People evacuate an area of Barcelona on August 17. Hide Caption 12 of 19 Photos: Terror attacks in Spain Medics and police tend to injured people near the scene of the attack in Barcelona. Hide Caption 13 of 19 Photos: Terror attacks in Spain A woman cries as she speaks on her phone in Barcelona on August 17. Hide Caption 14 of 19 Photos: Terror attacks in Spain People react after the incident in Barcelona. Hide Caption 15 of 19 Photos: Terror attacks in Spain A police officer asks people to move back in Barcelona. Hide Caption 16 of 19 Photos: Terror attacks in Spain An injured person is carried by police in Barcelona. Hide Caption 17 of 19 Photos: Terror attacks in Spain People flee the scene after the attack at Las Ramblas. Hide Caption 18 of 19 Photos: Terror attacks in Spain The Catalan emergency services urged people via Twitter to avoid going out or undertaking any other type of movement that is not "strictly necessary." Hide Caption 19 of 19

He also said that there had been a built-up police presence in the city for some time.

"It's a pretty obvious target. I noticed -- because I'm in and out of the city all the time -- I noticed a very, very large police presence. Check points, all over the city on the route to the airport, in the center, everywhere as kind of a show of force to, I guess, let the citizens and the tourists know that they are there and they are watching, and they are aware."

Witness 'saw at least three people on the ground'

London resident Charlie Parsons, 58, who was on vacation in Barcelona, was riding his bike when the attack happened about 20 meters or so away.

"I was on one side of Las Ramblas and suddenly a crowd of people came from my left screaming. Obviously, I felt something awful had happened," he told CNN. "It's a weird thing but I froze and didn't initially flee with them."

Parsons, who said he rarely goes to Las Ramblas, started walking in the same direction that everyone was running. He ducked in a shop as the shutters closed.

"Before I got into the shop, I saw at least three people on the ground. Clearly, they had been mowed down," he said. "I saw blood and didn't know what their condition was. I should have checked but what could I have done?"

In the shop, "people were shaking, kids screaming. It was complete panic," he said.

He said he then heard several gunshots, and one shot appeared to hit the metal shutters. About 10 minutes later, he left the shop through a back door.

"The streets were just weirdly ordinary and it was like people didn't know what had happened yet," he said.

'You are never going to feel safe in the world'

Valerie Istre, an American tourist from Dallas, Texas, was shopping on Las Ramblas avenue when her friend suddenly dropped and broke her sunglasses. Seconds after they walked into a clothing store to buy a new pair, the attackers struck.

"At first, we had no idea what was happening," said Istre. "We didn't speak the language. We knew it was bad and people looked scared. It was very surreal. You didn't know if it was just the van or if people were going to come in with guns."

Istre was immediately phoned by a cousin who works as a news reporter in Dallas. He was able to explain what was happening.

"Some of us gathered toward the back around a storage room with a lock on the door," Istre said. "The plan was to run inside and lock the door if needed."

Istre estimates there were 75 people in the store, mostly tourists. "People were crying, really freaking out. It was really scary."

Afraid they were going to die, many of the patrons began texting family members to say goodbye.

"I didn't know if I would ever see my three kids again," said Istre. "I sent my 12-year-old daughter a photo I had recently taken of me. I told her I loved her, but didn't say anything was wrong. I wanted her to have a nice, last picture of me in case something happened."

Istre remained holed up in the store with the other patrons for three hours until police came to escort them out.

"Had my friend not dropped her sunglasses, we could have been hurt. It was very, very, very surreal," said Istre.

"It makes you want to go home and hug your kids," she said. "But I also think it's this realization that you are never going to feel safe in the world we live in. It literally could happen any time and any place. You just don't know."