A van has ploughed into crowds on Las Ramblas in the central tourist area of Barcelona.
Police chief Quim Form confirms 13 dead, with 50 more people injured, some seriously.
A suspect has been arrested in the attack, a Moroccan national named Driss Oukabir, whose name was used to rent the van. However it is not known whether he was the man who was driving the van.
Local news station TV3 earlier suggested the driver was a Spanish citizen who lives in Melilla, a Spanish city which is situated in mainland Africa, bordering Morocco.
A local party in Melilla has reacted to the attack by saying "such a terrorist act and atrocity warrants our greatest revulsion and condemnation. Support to all the victims and families."
Dozens of people were hit by the van as it mounted the pavement of Las Ramblas, Barcelona's biggest tourist landmark, which was packed in mid-afternoon.
TV3 also reported that two armed people are locked in a restaurant with hostages, close by Las Ramblas, but police in Barcelona have rejected this report as untrue.
One witness who spoke to TV3 said "The van went at high speed and hit everyone it could. I saw people flying through the air and landing with blood.
"Las Ramblas was full, completely full. It was amazing."
Spanish police have confirmed they are treating the incident as a terror attack. A number of nearby landmarks and attractions have been closed, including the world-famous Boqueria market.
More to follow