One person was killed and 110 injured, five of them seriously, when two metro trains collided during the morning rush hour in the Italian capital, Rome.
The crash, on metro line A, took place at Piazza Vittorio Emanuele II station.
Passengers said the incident occurred when one train arriving at the station smashed into the back of another.
There have been reports that the moving train had passed a red light. Its driver, who was injured, has told police he has no memory of the event.
The crash is the first fatal accident ever to hit the Italian metro system.
The crash happened at 0937 (0737 GMT), one stop away from the mainline train terminus in Rome.
One train was stopped at the station platform to let passengers get off when the second train crashed into it from the back, leaving its front carriage concertinaed, passengers said.
"I saw the train in front and it seemed as though it was getting closer and closer to us and nothing was happening," Fabiano De Santis, a lawyer, told Italian television.
"I realised there was going to be an impact and so I managed to move forward in the carriage and I saw the train came towards me. It was a very strong impact."
The BBC's Christian Fraser in Rome says police investigators are focusing on the second train, which was new and had only recently been introduced to the network.
The passenger who died was a 30-year-old woman, officials said. Five of the injured people are in intensive care in hospital.
Rescue efforts were hampered by the dust and smoke and the loss of all lights at the station, but everyone on board the trains has been freed from the wreckage, reports say.
Shock and disorientation
Italian television showed images of victims being carried out on stretchers while other passengers emerged looking dazed and blood spattered.
"We saw people streaming out of the entrance to the tube station," Francesco Quirinis, a porter with the Hotel Napoleon, opposite the metro entrance, told the BBC News website.
EUROPEAN TRAIN ACCIDENTS
October 2006: Five killed in collision between two trains in north-eastern France
September 2006: A German monorail train crashes during a test run killing 23 people
August 2006: A train derailment kills six and injures 36 in northern Spain
June 2006: Forty-one people are killed when a metro train derails in the Spanish city of Valencia
January 2006: At least 44 people are killed and more than 180 injured when a train plunges into a ravine in southern Montenegro
"They looked shocked, disorientated, they were supporting each other. The police, ambulances - everyone was on the scene within 10 minutes and they immediately blocked off the piazza. There was a continuous coming and going of ambulances for about an hour after the crash.
"We gave them bottles of water, a place to sit down. We did all we could to help them - as anyone would."
The city's mayor later visited the crash site.
"When I arrived at the scene of the crash and saw it, it was difficult to describe. It is a terrible tragedy," Walter Veltroni told Italian television.
"Obviously, we don't understand why this happened - they are new metro trains so therefore in absolute working order. We have now asked the metro to try to give us the necessary information in order to help understand how something like this could have happened."