Some 15 ambulances rushed to the scene of the accident, which occurred near the Termini and Victor Emmanuele II stations, near Rome's central train station.
The all-news television station Sky TG24 showed images of victims being carried out on stretchers while other passengers emerged looking dazed.
ATAC, the Rome metro operator, confirmed reports of the crash at Piazza Vittorio underground station in the city centre but could not provide any more details on the extent of the damage or injuries.
Early reports said passengers were still stuck in some of the cars.
A passenger told Sky TG24 one metro train rear-ended another that was stopped.
Another witness said overhead beams of the station collapsed when one train rammed into the rear of another.
There were also reports that a electrical panel exploded when the trains crashed.