But officers were not amused six days ago when, in an earlier confrontation with Mr. Cercelletta at the fountain, he took a knife and made several horizontal slashes across his belly in protest of their intensified efforts to get him out of the fountain and separate him from his bags of change. They did not arrest him then.

In fact, officers have never really known what to make of him, or what to do with him.

For a long time, it was not clear that he was breaking any laws.

The coins thrown into the fountain were not really anyone's property, so Mr. Cercelletta was not guilty of theft, even though his predawn collection meant that when city workers went to the Trevi once a week to retrieve coins, there were many, many fewer than there should have been. Charities were deprived of at least tens of thousands of dollars every year.

But in 1999, a new Italian law to protect city monuments included a prohibition on wading into fountains like the Trevi and a financial penalty for doing so. So on those relatively rare occasions when police officers caught Mr. Cercelletta in the act, they issued him a fine. Then they issued him another, and another after that.

Mr. Cercelletta ignored them all, insulated from further punishment by his lack of a permanent address, his history of mental illness, the obvious ambivalence of some police officers and, most astonishingly, the absence of any evidence that he was holding onto much of the money or had the resources to pay. Mr. Cercelletta seems to have few possessions beyond the two primary staples of Roman life: a moped and a cellphone.

Antonio Del Greco, a senior police official, said that until the last week, when police officials began carefully monitoring Mr. Cercelletta, they had not considered the size of the sum that all of those submerged coins represented.

''You know,'' Mr. Del Greco said, ''I think nobody really realized that the fountain could be that fruitful.''