Authorities are treating the incident as a terrorist attack
Catalan emergency services say at least 13 people are dead and dozens injured after a van crashed into a crowd in Barcelona.
A white van mounted the pavement of the main tourist area in the city and struck several people, police said.
Authorities on Thursday evening confirmed 80 people were injured, 15 seriously. The number of injured was later increased to more than 100. The figure was expected to rise.
En estos momentos podemos confirmar 13 víctimas mortales y más de 100 heridos— Mossos (@mossos) August 17, 2017
Esta cifra puede subir en próximas horas@interiorcat
Officials confirmed one man was arrested in connection with the attack, and later said a second person had also been detained.
21:40 Police presence for the incident of #Barcelona is still working. We confirm two person under arrest— Mossos (@mossos) August 17, 2017
One social media video post shows bodies strewn across the pavement for several hundred metres along the famous street.
The Ministry of the Interior said on Twitter: "There has just been a massive crash on the Ramblas in Barcelona by a person with a van. There are injured."
Three days of mourning have been declared in Catalonia.
Both Taoiseach Leo Varadkar and President Michael D Higgins have expressed solidarity with the people of Barcelona.
Emergency services in Catalonia say they have asked the Metro and train services in the area to close.
They have also cordoned off the wide street and shut down several of the shops. They have evacuated buildings in the area.
Social media footage showed many people fleeing the area in alarm and a helicopter flying overhead.
Police have told people to avoid Plaza Catalunya area after the smash in Las Ramblas.
The Department of Foreign Affairs says it is monitoring developments closely through the Irish embassy in Madrid and Ireland's honorary consul in Barcelona.
It says: "Irish citizens in the vicinity of the incident are advised to follow the advice of local authorities."
Anyone with concerns for the safety of Irish people in Barcelona can contact the department in Dublin on +353-1-4082000 or the Irish embassy in Madrid on +34-91-4364093.
Minister Simon Coveney added: "I am appalled by the incident in Barcelona this afternoon.
"On behalf of the Irish government, I wish to convey our condolences and solidarity with the people of Spain at this time."
US President Donald Trump also condemned the attack:
The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you!— Donald J. Trump (@realDonaldTrump) August 17, 2017
An eyewitness, lawyer Aamer Anwar, who's at a conference in the city, said: "All of a sudden I heard a crashing noise, and the whole street just started to run screaming.
"I saw a woman next to me screaming for her kids."
Police are also asking people not to post images of the attack on social media.
We are aware of an incident in Barcelona and strongly advise caution. Anyone concerned can contact us at 4082000 @IrlEmbMadrid— DFATravelWise (@dfatravelwise) August 17, 2017
The Spanish Prime Minister Mariano Rajoy says he is contact with agencies following the incident.
En contacto con todas las administraciones. Prioridad: atender heridos en Las Ramblas y facilitar la labor de las Fuerzas de Seguridad. MR— Mariano Rajoy Brey (@marianorajoy) August 17, 2017
Additional reporting by IRN, Stephen McNeice