VALENCIA, Spain (Reuters) - At least 34 people were killed and more than 20 seriously injured when a speeding underground train ran off the tracks and overturned in the eastern Spanish city of Valencia on Monday, officials said.

Two train carriages derailed and toppled over in a tunnel just before entering the Jesus underground station after a wheel broke on a curve at about 1 p.m., officials said.

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

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 accident took place days before Pope Benedict was due to visit Valencia for the 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.

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 survivor in the last carriage described how he climbed out of a door window.

"There was a bang, some very strange noises, everything shook and I passed out. They told me when the firemen came. I don't know how long they took, but that's when I got up," the man told Cadena Ser radio.

Prime Minister Jose Luis Rodriguez Zapatero was considering breaking off an official visit to India and King Juan Carlos rang Valencia's government to offer his condolences, Spanish media reported.

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

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

In June 2003, 19 people were killed when a passenger train travelling to Cartagena from Madrid crashed into the path of an oncoming goods train at Chinchilla.