A government official tweeted that 13 people have died and at least 50 have been injured.
Police said a “massive crash” occurred when a van hit crowds in the Ramblas tourist area.
Witnesses said the van “zigzagged” down the avenue mowing down pedestrians.
Police said they have arrested a man in connection with the attack, but did not identify him.
They were previously searching for the driver of the van, who Spanish media said escaped the scene on foot.
Spanish media earlier reported that two armed men were entrenched in an establishment near the area of the attack, however, police refuted these reports.
The Catalan emergency services said on Twitter that there had been an incident near Placa Catalunya and advised people to avoid going to the area.
Police have asked for the metro and bus stations in the area to be closed. (Euronews)