Four carriages overturned in the smash, smoke billowing from the remnants, as bodies were lain out under blankets along the tracks.
The wagons piled into each other and folded up like an accordion. One was ripped apart by the force of the smash, one of its ends pushed upwards into the air.
Disaster struck at 8.42 pm (1842 GMT) as the train carrying 218 passengers and four staff was about to enter Santiago de Compostela station in the northwestern region of Galicia.
The last official numbers being reported by Spanish media outlets have the death toll at 77 people.
This makes the train accident Spain's worst in four decades.
Francisco Otero, 39, who was inside his parents' home just beside the section of the track where the accident happened, said he "heard a huge bang. As if there had been an earthquake."
"The first thing I saw was the body of a woman. I had never seen a corpse before. But above all what caught my attention was that there was a lot of silence, some smoke and a small fire," he told AFP.
"My neighbours tried to pull people who were trapped inside the carriages with clubs and they eventually got them out with a hand saw. It was unreal."
The train had left Madrid and was heading for the northwestern town of Ferrol as the Galicia region was preparing celebrations in honour of its patron saint James.
A witness told radio Cadena Ser that carriages overturned several times on a bend and came to a halt piled up on each other.
Public television TVE said the train may have derailed because it was speeding at the time of the accident but a spokesman for state railway company Renfe said it was too soon to say what caused the accident.
"There is an investigation underway and we have to wait. We will know what the speed is very soon when we consult the train's black box," a Renfe spokesman said.
The accident happened on a stretch of high-speed track about four kilometres (2.5 miles) from the main train station in Santiago de Compostela, the destination of the famous El Camino de Santiago pilgrimage which has been followed by Christians since the Middle Ages.
The train — described by El Pais newspaper as practically new — was the Alvia model which is able to adapt between high-speed and normal tracks.
Emergency services workers in red jackets tended to injured passengers lying on a patch of grass as ambulance sirens wailed in the background.
"There are bodies laying on the railway track. It's a Dante-esque scene," Feijoo told news radio Cadena Ser.
Carriages were lying across the tracks, some of them jammed alongside a concrete siding.
Prime Minister Mariano Rajoy, who is from Santiago de Compostela, was to visit the scene of the accident on Thursday.
"I want to express my affection and solidarity with the victims of the terrible train accident in Santiago," he said in a twitter message.
Pope Francis called for prayers for the victims of the accident.
"He joins the families in their sorrow and calls for prayers ... in this tragic event," Vatican spokesman Federico Lombardi told reporters during the pope's visit to Rio de Janeiro.
The town hall of Santiago de Compostela called off planned concerts and firework displays that had been planned as part of the festivities in honour of its patron saint.
The crash came less than two weeks after a passenger train derailed just south of Paris, killing six people and injuring 30 more.
French rail operator SNCF said that derailment may have been caused by a connecting bar that came loose.