The "massive crash" is now confirmed to have been a terrorist attack.
A white van mounted the pavement of Las Ramblas and struck several people, Sky News reports.
The occupants of the van are reported to have fled into the nearby streets.
Officials have confirmed there have been fatalities, with Spanish radio station Cadena Ser Radio and El Pais newspaper reporting that 13 people have died.
Images posted on social media showed armed police swarm the area popular with holidaymakers.
The Foreign Office said in a statement on its website: “The British Embassy in Madrid and Consulate General in Barcelona are in contact with local authorities and urgently seeking further information following reports of an incident in central Barcelona; local authorities have advised people to stay inside and stay away from the Las Ramblas area of the city.
“If you’re in the immediate area you should take care and follow the advice of the local security authorities.
“Press reports suggest that some public transport, particularly the metro, has been affected.”
The National Police have told people to avoid Plaza Catalunya area.
