Catalonia's regional head said on Thursday that Spanish police had arrested two people after a van drove into crowds in Barcelona city's centre.
At least 80 people have been hospitalized and around 12 have died, Carles Puigdemont told a news conference. It was still not clear how many attackers had been involved in the incident.
Puigdemont said people had been flocking to hospitals in Barcelona to give blood.
Barcelona's mayor, Ada Colau, said the city would hold a minute of silence on Friday at midday.