The crash, believed to be the first fatal accident ever to hit the Italian capital's metro system, occurred during the morning rush hour.
Some 15 ambulances rushed to the scene of the crash, taking the most seriously injured to hospital. Among them was a French tourist who was bleeding badly from a head wound.
Several dozen people with minor injuries were seated around the entrance to Vittore Emanuele II station awaiting treatment, while many women were in shock and weeping. A rescue official told AFP that 101 people were injured in all.
They included the driver of the locomotive that hit the stationary train, a hospital official said.
Doctor Adolfo Pagnanelli said driver Angelo Tomei had been questioned by a magistrate but had no memory of the accident. According to unconfirmed reports he may have gone through a red stop light at the previous station.
Although he was in deep shock, the driver was expected to be discharged from hospital in a few days, the doctor added.
Passenger Simone Saltarelli told the ANSA news agency said the second train had left the previous station "when there was a red light."
"I think you're supposed to wait for the green light, but this time I'm sure I saw the red," he said.
But Rome administrator Achille Serra said: "It would not be opportune to speculate now on the causes of the accident. A magistrate is going to the scene and will make a judgement following his investigation."
The moving train was Spanish made, and one of the latest put into circulation to replace Atac's current fleet.
The accident occurred at around 9:30 am at the Victor Emmanuele II station on the busy Line A. Rescue operations were completed shortly after 11:00 am.
"I was in the metro train that was stopped at the station when I saw the other train run into us. I was horribly frightened," a passenger who gave his name only as Alberto told AFP. "I thought the train would crash on top of us."
Security services sealed off the area, and police manned the station's four entrances. Line A was stopped following the accident, which wreaked havoc on Rome traffic.
Sky TG24 showed images of victims being carried out on stretchers while other passengers emerged looking dazed.
Italian President Giorgio Napolitano sent a message of condolences to the families of the victims, saying he was "very affected by the tragedy."
A list of earlier accidents on the Rome subway system released by the ANSA news agency showed no previous fatalities on the network.

Copyright AFP (Agence France-Presse), 2006