Two train carriages overturned in the tunnel just before entering the Jesus Metro station, apparently after a wheel broke on a curve, officials said. About 150 people were evacuated from the station platform, Reuters reports.

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.

The accident took place days before Pope Benedict was due to visit Valencia for the World Meeting of Families and pilgrims are already arriving in the Mediterranean seaside city.

"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, adding that the number of dead was still not confirmed.

A senior police officer at the scene said 34 people had been killed.

Police sealed off streets outside the station and crowds watched under bright blue skies as emergency crews rushed injured people into ranks of ambulances.

One woman, her face blackened by what looked like soot, grimaced with pain as she walked with her arm around a police officer.

Emergency services set up two field hospitals in tents on the street and a judge arrived to supervise the removal of bodies. Hospitals appealed for blood donors.

The Pope has been informed of the accident and went to his private chapel to pray for the victims, Spanish state radio reported.

The mayoress of Valencia Rita Barbera told reporters the accident was "a catastrophe, a tragedy".