The device was found on an empty carriage in a train at Rebibbia station on the outskirts of Rome [Reuters]

A suspected bomb has been found on an underground train carriage in Rome, Italy's capital, amid growing concern over attack plots during the holiday season in Europe.

The package, discovered on Tuesday morning inside a train at Rebibbia station, had wires, pipes and explosive powder, Italy's Ansa news agency said.

However bomb disposal experts told police that the device could not have blown up because it lacked a detonator.

"It didn't have a detonator, it couldn't explode," a spokesman for the Carabinieri, Italy's paramilitary police force, told the Reuters news agency.

Gianni Alemanno, Rome's mayor, said the discovery was "worrying".

"Bomb disposal experts are looking into what it is," he said.

Reports said the device had been left in a grocery bag underneath a seat on the train, and was found by the conductor.

Heightened alert

The development comes at a time when many European countries are on heightened alert over plots in the region.

Sabina Castelfranco, Al Jazeera's correspondent in Rome, said the device triggered alarm bells in the city.

"This is a difficult and busy period in Rome, with Christmas coming up in just a couple of days and also a huge demonstration planned tomorrow to protest against an education reform bill.

"There are concerns that such devices could have been used possibly tomorrow or by terrorists, " she said, adding police were investigating who could have been behind the device.

Iraqi officials claimed last week that captured fighters believed that the suicide bombing in Stockholm on December 11 that killed only the bomber was part of a series of holiday season attacks.

On Monday British police arrested 12 men in a counter-terrorism raid over a possible home-grown plot.

Both British and German officials have insisted there have been no new specific threats to their countries over the festive period.

Britain's terror alert has remained unchanged at "severe'' while Germany upped its terror alert on November 17, when Interior Minister Thomas de Maiziere warned of an increased threat from "Islamic extremists".

Source: Al Jazeera and agencies