JOIN USThe device was made up of two short metal tubes stuffed with gunpowder that were connected by wires and had an antenna, but there was no detonator.
It was found by a metro transport worker at around 10:00 am (1430 IST) in a service area near Rebibbia station in the northeastern outskirts of the city.
"It could have been a warning action. The device was well put together. It was done by an expert," Maurizio Mezzavilla, the head of the Carabinieri paramilitary police force in the Italian capital, told reporters.
Security measures have been stepped up around Rome following a violent protest against Prime Minister Silvio Berlusconi in the city centre last week and ahead of major student rallies expected tomorrow.
Prosecutors have opened an inquiry on possible charges of possession and handling of explosive material and police officers were analysing CCTV footage to see how the bomb was brought onto the train, officials said.
The train started out from Laurentina station in southwest Rome, passing through the city centre and ending up at Rebibbia in northeast Rome.
Giampaolo Polizzaro, a spokesman for Rome city council, said the device was inside a blue box in a plastic bag and "could not have exploded."
The website of the Corriere della Sera newspaper quoted a police source saying it could have been a pipe bomb -- a popular type of improvised device.
Rome mayor Gianni Alemanno said the news was "worrying" and he congratulated the security forces for their quick reaction to the alert.Alemanno said security officers had checked all the stations on Rome's metro system after the incident and found no other type of suspect packages.
The bomb was found under a seat after the last passengers had descended, Rome's transport authority ATAC said in a statement.Officers were called "immediately" and isolated the area where the bomb was found but the station was not evacuated, ATAC said.The metro system continued functioning as normal during the operation.