At least 68 people have been killed after a Spanish high-speed train collided with another train south of the capital Madrid, with dozens more seriously injured.

Speaking from the crash site, Spain's Prime Minister Pedro Sánchez says "the whole of Spain is grieving", adding: "Lives have been shattered, families have been torn apart, the pain is immense."

Passenger Eduarda was travelling with her two cats and told Hoy por Hoy, a Spanish radio programme, that after the collision she heard screams and saw a carriage on fire, with people under the train who had died.

Transport Minister Óscar Puente says a computer failure led to the initial halt of six moving trains, including the two trains that subsequently collided.

"In the event of a system failure, the control centre takes over," Puente says, adding it was the control centre which decided to allow the trains to move again, without any speed restrictions.

That decision, Puente said, was made "under inexplicable circumstances", and resulted in the collision.

An investigation is under way, dozens are being treated in hospital and three days of mourning have been announced by the prime minister.