by Elisa Forte and Gavin Jones

POTENZA, Italy (Reuters) - Italian police said on Friday they had seized about $6 trillion worth of fake U.S. Treasury bonds and other securities in Switzerland, and arrested eight Italians accused of international fraud and other financial crimes.

The operation, co-ordinated by prosecutors from the southern Italian city of Potenza, was carried out by Italian, Swiss and U.S. authorities after a year-long investigation, an Italian police source said.

It began as a investigation into mafia loan-sharking, but gradually expanded as prosecutors used telephone and computer intercepts to unearth evidence of illegal activity surrounding Treasury bonds.

The fake securities, worth more than a third of U.S. national debt, were seized in January from a Swiss trust company where they were held in three large trunks.

The U.S. Embassy in Rome thanked the Italian authorities and said the forgeries were "an attempt to defraud several Swiss banks." It said U.S. experts had helped to identify the bonds as fakes.

Potenza's prosecutor Giovanni Colangelo said an international network "in many countries" was behind the forgeries.

Italian daily Corriere della Sera said on its website that the criminal network was believed to be interested in acquiring plutonium, citing sources at the prosecutors' office.

VERSAILLES

Police videos showed images of the trunks, with "Federal Reserve System, Treaty of Versailles" stamped on the side in large, golden letters.

Bond certificates marked "Chicago, Illinois, Federal Reserve Bank" and other securities, some for one billion dollars, were also shown.

U.S. bond traders took a light-hearted view of the news.

"If there's that much less supply now, Treasuries should be rallying," joked Kevin Flanagan, fixed-income strategist at Morgan Stanley.

A trader at Citigroup said he had swapped jokes with colleagues about the seizure, which would not move markets.

"It's kind of like fake inflation I guess, if you take it to the max, but I don't think it means that much."

Prosecutors said the forgers had hoped to use the fake bonds as collateral to secure loans.

The eight men arrested are accused of counterfeiting bonds, credit card forgery, and loan-sharking in the Italian regions of Lombardy, Piedmont, Lazio and Basilicata, police said.

The Swiss Federal Prosecutor's office said Zurich state prosecutors had worked on the investigation at the request of the Italian prosecutor. The Swiss handed over their findings in July last year.

In 2009, Italian financial police seized $742 billion of fake U.S. bearer bonds in the of Chiasso, on the Swiss-Italian border.

(Additional reporting by Steve Scherer and Emily Flitter; Editing by Andrew Roche)