The accident happened at 7.45pm (local time) on Sunday near Adamuz in the province of Cordoba, about 360km south of the capital Madrid.
Over 200 trains between Madrid and the southern Andalucia region - including major cities Cordoba, Seville and Granada - have been cancelled throughout Monday, according to RTVE.
Video from the scene shared on social media on Sunday showed rescuers pulling passengers from twisted carriages lying on their side under the glare of floodlights. Some passengers climbed out of smashed windows, while others were wheeled away on stretchers.
There were around 400 passengers on the two trains, most of them Spaniards travelling back to and from Madrid after the weekend. It was unclear how many tourists could be onboard as January is not holiday season in Spain.
The cause for the crash is not yet known, Spanish Transport Minister Oscar Puente told reporters at a press conference at Atocha station in Madrid on Sunday.
Horrific scene
Spanish Prime Minister Pedro Sanchez cleared his diary for Monday to address the tragedy, while the Spanish King and Queen were following events with concern, a spokesperson said. Foreign embassies sent text messages to staff asking them to confirm they were safe.
Troops have also been deployed from a military base near the crash site to support rescue and recovery efforts, the Military Emergency Service confirmed.
The mayor of Adamuz, Rafael Moreno, told El Pais newspaper that he had been among the first to arrive at the scene of the accident alongside the local police and saw what he believed to be a badly lacerated body several metres from the accident site.
"The scene is horrific," he said. "I don't think they were on the same track, but it's not clear. Now the mayors and residents of the area are focused on helping the passengers."
Tearful passengers
Images on local television showed a reception centre set up for passengers in the town of Adamuz, population 5000, with locals coming and going with food and blankets amid night-time temperatures of around 6°C.
Tearful passengers disembarking from the bus spoke briefly to local press before being shepherded inside.
"There are many injured. I am still trembling," Maria San José, 33, a passenger in coach 6 on the train travelling from Malaga to Madrid, told El Pais.
A passenger from the Alvia train, who was not named, told TVE: "There were people screaming, their bags fell from the shelves. I was travelling to Huelva in the fourth carriage, the last, luckily."
A woman named Carmen posted on X that she had been on board the Iryo train to Madrid. "Ten minutes after departing (from Cordoba) the train started to shake a lot, and it derailed from coach 6 behind us. The lights went out."
Footage posted by another Iryo train passenger, also on X, showed an Iryo staffer in a fluorescent jacket instructing passengers to remain in their seats in the darkened carriages, and those with first aid training to keep watch over fellow passengers. He also urged people to maintain mobile phone batteries to be able to use their torches when they disembarked.
Salvador Jimenez, a journalist for RTVE who was on board the Iryo train, shared images showing the nose of the rear carriage of the train lying on its side, with evacuated passengers sitting on the side of the carriage facing upwards.
Jimenez told TVE by phone from beside the stricken trains that passengers had used emergency hammers to smash the windows and climb out, and they had seen two people taken out of the overturned carriages on stretchers.
"There's a certain uncertainty about when we'll get to Madrid, where we'll spend the night, we've had no message from the train company yet," he said. "It's very cold but here we are."
