A truck hit pedestrians on Catalunya Square in Barcelona's Las Ramblas district this afternoon.
Spanish authorities said there were injuries but no deaths have been confirmed.
Las Ramblas is a popular tourist area in Spain's second-largest city.
The Barcelona transit system posted a message on its official Twitter account that numerous subway stations are closed and it urged people near the area to remain in place until further instructions from police.
The U.S. Consulate in Barcelona posted a warning on Twitter, saying it is "aware of a reported incident at Las Ramblas in Barcelona. Please avoid the area."
The emergency services Twitter account for the Catalonia region, of which Barcelona is the capital, posted tweets in multiple languages advising individuals to use social media to inform relatives of their whereabouts.
This is a breaking news story. Please check back for updates.