Emergency teams worked through the night to find victims after one of the worst rail disasters in Europe's modern history. By Harry Sekulich Spain is beginning three days of mourning after at least 68 people were killed and dozens more were injured in one of the worst rail disasters in Europe's modern history.

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.

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

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".

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.

The stationary train, run by private rail operator Iryo, had 147 passengers on board, and had stopped on the tracks because of a computer failure, according to Spain's transport minister Óscar Puente.

He said that when there is a system failure, a 24-hour control centre in Madrid takes over, but it had made an "inexplicable" decision to instruct the trains to move again, without any speed restrictions.

Audio recordings between train controllers in Madrid and the train drivers appear to show that one high-speed train was given clearance to move even though another up ahead had stalled.

Iryo said its train was "struck from behind" by another train, run by Spain's state-owned railway company, Renfe, which was carrying 397 passengers. The force of the crash caused several carriages to fall down an embankment.

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 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 when 80 people died on the outskirts of Santiago de Compostela in north-west Spain.