Las Ramblas, a street of stalls and shops that cuts through the center of Barcelona, is one of the city's top tourist destinations.
A white van mounted the footpath and struck several people in the area in what police said was a "massive crash" and have described it as a "terrorist attack".
Details are still coming in.
It is unclear how many people have been injured at this time.
They asked people to stay away from the area so as not to get in the way of the emergency services.
Local media has reported that two people have died and 20 injured and that attackers were holed up in a bar.
Moments ago, two armed men entrenched themselves in a bar in Barcelona's city centre after a van mowed down dozens of people, El Periodico newspaper reported.
Social media users say that the suspect fled into a nearby restaurant, took hostages and opened fire.
A police officer cordon off a street in Barcelona, Spain, Thursday, Aug. 17, 2017.
The driver abandoned the vehicle and fled on foot, police said.
The street is one of the most well-known and popular parts of the city, visited by tourists and locals alike. Esparcia said he was taking cover inside a Starbucks.
Eyewitness Amar Anwar, human rights lawyer, told Sky News: "I did not see the van mount the pavement".