At least 41 people were killed when an underground train ran off the tracks and overturned at high speed in the eastern Spanish city of Valencia on Monday, officials said.
They said a wheel broke on a curve as the train approached the Jesus underground station and two train carriages derailed in the tunnel. Another 47 people were hurt, of whom 12 remain hospitalised, two of them in critical condition.
Rescue teams worked into the night to recover the dead from inside wrecked carriages.
"There may be other bodies, forensic police are working intensively at the accident scene," said central government official Antonio Bernabe, who put the number of dead at 41.
Trapped passengers rang emergency services from mobile phones and 150 people were evacuated from the station platform. One middle-aged woman, her face blackened by what looked like soot, grimaced with pain as she staggered away from the station, her arm around a police officer.
Officials ruled out a terror attack in a country still shaky from train bombings by Islamist militants that killed 191 people in Madrid in 2004.
"It seems this unfortunate accident was caused by excess speed and a wheel breaking just before it entered the station," government official Luis Felipe Martinez told Spanish radio.
Emergency services set up two field hospitals in tents on the street and a judge arrived to supervise the removal of bodies. Hundreds of people converged on the city's morgue to help identify victims.
Vicente Rambla, a spokesman for the Valencia regional government, said such work was difficult as much of the human remains were "unrecognizable".
The accident took place days before Pope Benedict was due to visit Valencia for a 'World Meeting of Families' and pilgrims were already arriving in the Mediterranean seaside city.
Thousands of yachting enthusiasts were also visiting Valencia, which is staging warm-up races for the Americas Cup.
Spanish Prime Minister Jose Luis Rodriguez Zapatero cut short an official visit to India and will attend a funeral ceremony in Valencia on Tuesday evening.
Faster than usual
Police sealed off streets outside the station and crowds watched under bright blue skies as emergency crews rushed injured people into ranks of ambulances.
A 21 year old student, Cesar Hernandez, said the train began travelling faster than usual and was shaking from side to side before the train braked suddenly and the carriages derailed.
Hernandez kicked the glass out of a door and walked out into the tunnel.
"There wasn't much light and I couldn't see much of what was on the tracks. I saw people on the ground, but I just ran," Hernandez told newspaper El Mundo, adding that he had declined emergency service offers of trauma counselling.
"I didn't want psychological help or anything. I just wanted to see my Dad," he said.
In September last year, three trains crashed into each other in Valencia's Metro system, injuring 16 people.
Spanish consumer group FACUA called for an investigation into the safety of the Metro's Line One.
The Pope went to his private chapel to pray for the victims, state radio said.
"The Holy Father was immediately informed of the tragic accident in Valencia and followed the dramatic news from that city with pain and sharing," said Vatican spokesman Joaquin Navarro-Valls.