Audio recordings published in Spanish media reveal conversations between train drivers and a railway control centre in the minutes leading up to a deadly train crash that killed at least 68 people and injured dozens.

Officials are investigating the cause of the disaster, which occurred when a train travelling from Madrid to Seville barrelled into a stationary train on Wednesday morning.

Three audio recordings between the control centre in Madrid and two train drivers appear to show that one high-speed train was given clearance to move even though another some 25km (15 miles) up ahead had stalled.

The release of the recordings comes as Spain begins three days of national mourning for one of the worst rail disasters in Europe's modern history.

The stationary train, run by operator Iryo, had stopped on the tracks because of a computer failure, at which point a 24-hour control centre in the Spanish capital took over.

The audio recordings have been published by Spanish news outlets including El Pais and RTVE.

In one recording, a control centre operator tells the driver of the train run by state-owned railway company Renfe to resume its journey at normal speed after some technical issues.

"Continue your journey and then proceed under [Lateral Signalling Block], at normal speed with normal operation," the control centre operator says to the driver.

The driver replies, "At normal speed?" To which the controller says: "Yes."

The Lateral Signaling Block is a back-up or alternative train system used when the automatic signalling or data transmission system fails.

Later, the driver of the other train, operated by private company Iryo, says in another audio recording that communication with the train-control system has been lost and they are stationary at Urda.

The train "is stationary in front of the entry signal at Urda", the driver tells the control centre operator. After a series of exchanges, the driver is then told to "resume the journey" normally.

The Iryo driver then contacts the control centre again, according to a later recording, saying they cannot start the train. "The train won't allow me to control the [security] system. That's why I am still stopped in front of the entry signal," they say.

The latter recording suggests that the controller had been notified that the Iryo train remained stationary as the Renfe train approached.

Iryo said its train, which had 147 passengers on board, was "struck from behind" by another train, by Renfe train which was carrying 397 passengers.

Spain's transport minister Óscar Puente said on Wednesday that the decision to instruct the trains to move again, without any speed restrictions, was "inexplicable".

Prime Minister Pedro Sánchez said the "whole of Spain is grieving", adding that "lives have been shattered, families have been torn apart, the pain is immense".

Emergency crews worked through the night under powerful floodlights to cut open metal carriages in the search for victims, with many of those killed in the crash still to be identified.

Hundreds of personnel have been deployed to the disaster site to assist with recovery efforts, according to Spain's interior ministry, with drones and helicopters being used to find victims and remove rubble.

Witnesses described seeing people crying and bleeding, passengers trapped in their seats and smoke billowing throughout the carriages, one of which caught fire.

Sánchez said it was "unacceptable" that Spain had suffered two serious rail disasters this year, after a high-speed crash killed 46 people near Córdoba in January. It occurred on the same line as Wednesday's accident, but much further south.

Most political parties suspended campaigning for upcoming elections following the incident.

The exception was the far-right party Vox. Its leader, Santiago Abascal, said his party would not take part in what he described as attempts to "use mourning to silence criticism" of the country's rail infrastructure.

Overnight, around 1,500 people gathered outside the Spanish Transport Ministry building in the capital, according to Spanish broadcaster TVE, which reported that demonstrators were largely calling for the resignations of Sánchez and Puente.

Police used tear gas to disperse crowds after protesters hurled bottles and flares. No injuries or arrests were reported.

Rail drivers are expected to go ahead with a planned strike on Thursday, with the Spanish Union of Railway Drivers (Semaf) saying the strike action will be held "in defence of railway safety".

The leader of Spain's main opposition People's Party Alberto Núñez Feijóo said: "Spain has the right to know what is happening with its infrastructure."

Late on Wednesday, relatives were still trying to gather information about passengers who were on either of the trains.

The Spanish Civil Guard has set up bureaus in Madrid, Toledo, Seville and Córdoba to investigate reports of missing persons.

"The top priority is the identification of the missing persons and the taking of DNA samples to be able to locate their relatives," the civil guard wrote in a post on X.

In Toledo, close to the site of the crash, locals observed a minute's silence during a vigil to commemorate the victims.

European Union flags are flying at half-mast in Brussels, in what the European Commission called a "sign of European solidarity" during Spain's mourning period.

Wednesday's crash is the worst the country has seen since 2013, when 80 people died on the outskirts of Santiago de Compostela in north-west Spain.