Official: Bomb Discovered in Rome Subway a 'Fake'

VIDEO: Italian police are taking the device seriously despite the lack of a
WATCH Suspected Bomb Found in Italian Train Station

A device suspected of containing explosive powder that was discovered today in the subway system in Rome was a "fake," the city's mayor said.

"The device could not have exploded," mayor Gianni Alemanno said, according to a report by The Associated Press. Alemanno said the package was "a fake."

One Italian news service said the device, which contained a powdery substance initially thought to be an explosive, was left in a grocery bag underneath a seat and was spotted by a conductor, the AP reported. The stop, which is located on the outskirts of Rome, was evacuated.

The discovery came just a day after 12 men were arrested in the United Kingdom as they were allegedly in the final stages of a major bomb plot targeting several cities there.

The men were arrested after weeks of surveillance by law enforcement and MI5 in raids in London, Cardiff, Birmingham and Stoke-on-Trent. The men are between 17 and 28 years old and are mostly British citizens from Bangladeshi and Pakistani backgrounds.

"The arrests were absolutely necessary to keep the public safe," John Yates, assistant commissioner of the Metropolitan Police and Britain's senior counterterrorism police officer, said Monday of the U.K. arrests. He also reminded people to remain vigilant due to a large number of terror threats that officials are currently monitoring.

Authorities did not say the plot was timed for the holidays, but they did feel it was necessary to act right away, an indication this plot was in its final stages. The arrests were the biggest anti-terror action in Britain since April 2009, when another dozen men were detained in Manchester in connection with an alleged Al Qaeda bomb plot.

U.S. Attorney General Exclusive: 'Threat Is Constant'

Earlier this year, the U.S. State department issued a travel alert for Europe because of a heightened concern about terror.

Today, U.S. Attorney General Eric Holder gave Americans a brunt warning about the continued danger of terror attacks, in an exclusive interview.

Holder told ABC News he's confident the U.S. will continue to thwart attacks on the American homeland, but he said "the terrorists only have to be successful once."

"What I am trying to do in this interview is to make people aware of the fact that the threat is real, the threat is different, the threat is constant," he said.

CLICK HERE to see the full exclusive interview.

Last week U.S. authorities sent a message to local law enforcement nationwide warning of possible attacks during the Christmas season because of their "psychological impact."

ABC News' Jack Cloherty and Pierre Thomas contributed to this report.