Officials at the Piazza Vittorio metro station, near the main overground rail terminal in the city centre, said there was no indication the accident was related to terrorism.
The accident happened when one train arriving at the station crashed into the back of another as passengers were still disembarking.
"About one quarter of the first carriage of the train I was on was totally crushed," one passenger said.
The injured, many of them spattered with blood, were taken to six hospitals in or near the city centre, causing traffic chaos.
"I was on the train that hit the other train. I saw everything," said Fabiano De Santis, a lawyer.
"I clearly saw a red light then luckily I managed to anticipate the situation because I saw the train before us."
Another witness said overhead beams in the station collapsed and the lights went out.
Transport Minister Alessandro Bianchi said of the 236 injured, 210 did not need hospital treatment. Five were seriously injured but their lives were not in danger.
Mr Bianchi said it was unlikely the train was travelling at high speed when it crashed, due to an automatic braking system which cuts in after trains exceed a certain speed.
"This means that maybe we should be thinking that the speed was fairly low, around 25 km/h," he said.
Investigation underway
Prime Minister Romano Prodi said officials were "still looking into whether it was a human mistake or if something went wrong with the signalling".
"The trains were brand new and there is no visible technical fault at the moment. Now let's give our condolences to the casualties and let's stay close to the ones who are in hospital," he said.
Rome Mayor Walter Veltroni said: "When I first heard of it, I was afraid it could be something else," in an apparent reference to fears of a terror attack.
Mayor Veltroni's office said he ordered the metro company to carry out an investigation and to include outside experts in its probe.