For decades of Christmases, it had been a gratifying way to function as a substitute Santa Claus. Every holiday season, thousands of New Yorkers trooped to Manhattan’s main post office and sifted through heaps of dream-encased letters that children had scribbled to the big guy at the North Pole. They picked out the ones that tickled the heart and responded with gifts for otherwise empty stockings.

Then came Thursday.

Gift-giving souls who reported to Operation Santa Claus at the post office on Eighth Avenue and 33rd Street, looking for the familiar cardboard boxes bursting with letters, were instead greeted with no boxes, no letters and no explanation.

The United States Postal Service abruptly shut down public participation in all the Operation Santa programs  in New York and other major cities across the country  at 1 p.m. Wednesday, without offering post offices or letter-seeking citizens any understanding of why.

A Postal Service official in Washington, after an initial, limited acknowledgment of a “privacy breach,” said that at one of the programs, not New York’s, a man whom a letter carrier recognized as a registered sex offender had “adopted” a letter. When postal officials confronted the man, the official said, he said he was sincerely trying to do a good deed, but postal inspectors nonetheless retrieved the letter and notified the family of the child.