Rescuers are still searching for missing revellers after the blaze broke out in the city of Murcia in the early hours of Sunday morning.
Another four people have been injured after suffering smoke inhalation at the Atalayas nightclub, which is said to be made up of three different adjoining venues – Golden, Teatre and Fonda.
Outside the club, young people hugged, looking shocked as they waited for information. Spanish media reported several birthday celebrations were taking place at the time the fire broke out.
“I think we left 30 seconds to 1 minute before the alarms went off and all the lights went out [and] the screams saying there was a fire,” one survivor, who was not identified, said. “Five family members and two friends are missing.”
Diego Seral, of Spain's National Police, told reporters the dead were found in Fonda, which had sustained the majority of fire damage, including the collapse of its roof. The collapse was making it difficult to locate victims, and the identification of the bodies would take time, he said. The emergency services gave the death toll, which has risen steadily throughout the day, as 13.
Murcia’s Mayor Jose Ballesta declared three days of mourning for those who had died. Flags were lowered to half-mast outside Murcia's City Hall.
“We are devastated,” Mr Ballesta said on Spanish TV channel 24h, adding rescuers were still searching for several people reported missing.
The cause of the blaze is being investigated.
Mr Seral said that it was difficult to pinpoint yet where exactly the fire started. A spokesperson for the Teatre nightclub, Maria Dolores Albellan, told reporters the fire originated in the neighbouring club, La Fonda, before spreading to the two adjoining clubs.
Footage released by Murcia's fire service showed firefighters working to control flames inside the nightclub. The fire had destroyed part of the roof, the footage showed.
The Murcia Fire and Rescue Service said 40 firefighters and 12 fire engines were at the scene shortly after 6am (UK time) tackling the blaze.
Mr Ballesta told 24h the fire started at around 6 am and had now been brought under control.