Rescue workers were scouring wreckage for more bodies at the crash scene on Wednesday in the province of Toledo.
Prime Minister Pedro Sanchez said "all of Spain" was in mourning after the crash, which Transport Minister Oscar Puente said happened after a Renfe service was authorised to proceed despite an active red signal due to a stationary train ahead.
Puente told reporters in Madrid that the red signal was issued due to a computer communications failure, which automatically stopped traffic, but an erroneous decision was then taken to restart the Renfe train.
At the scene in rural Castilla-La Mancha, ambulances, funeral home vans and helicopters assembled beside one of the trains on an elevated stretch of track while twisted metal and wreckage lay scattered down an embankment where at least three carriages had plunged.
The head of the regional government Emiliano García-Page said the death toll had risen to 68 while rescuers searching for other victims as darkness fell did not know if other fatalities would be discovered once the carriages were moved.
Passengers removed from the train streamed along a road near the scene, some splattered with blood or with their wounds bandaged, others wheeling suitcases and being given water by volunteers who said it had been donated from neighbouring towns.
Miguel, a passenger travelling with several colleagues in the rear car of one of the trains, described to COPE radio the moment of the accident.
"We heard a massive roar, a truly enormous thud that went on for 10 seconds, as if the train were about to derail. Through the right-hand window, we could see a wrecked train," he said.
The disaster, the deadliest train accident in Europe since a 2013 derailment in Spain's Santiago de Compostela in which 80 people died, follows another fatal accident in January involving high-speed trains operated by the same companies on the same line, in which 46 people died.
Renfe said on Wednesday that after being authorised to continue its journey, its train had crashed into the Iryo train that had remained stationary "for reasons we don't know".
"The Renfe train continued on its way as instructed, and a collision occurred. The exact sequence of events, the traffic conditions and the circumstances of the impact will have to be established by the investigation," Renfe said in a statement.
Iryo, which said it had activated emergency protocols, and state-owned infrastructure operator Adif did not respond to requests for comment.
A Castilla-La Mancha government spokesperson said 147 people, including a baby, were on board the Iryo train and 397 on the Renfe service when the crash occurred about 9am, derailing two Iryo carriages.
Passenger Rafael Barcelo said he felt lucky as he had stepped away from his seat for a coffee, adding there had been at least eight victims from his carriage.
"We were having breakfast, drinking a coffee when we felt a very strong impact and we were sent flying," he said.
Renfe said it had opened a special line for families of victims.
The railway operator said it had suspended ticket sales on the Madrid-Andalusia high-speed corridor until December 13.
Spanish Health Minister Monica García said emergency workers were focusing on rescuing survivors and bodies from the two most damaged train cars.
The crash, which occurred in a flat stretch surrounded by olive fields in between the towns of Urda and Los Yebenes, left one car shredded into bits of metal and other material, scattering debris down a gravel slope.
The accident was between two high-speed trains heading in the same direction from the capital Madrid to Seville in southwestern Spain, according to Esther Padilla, spokeswoman for the Castilla La Mancha region.
with AP
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