The work, “Girl With Balloon,” a 2006 spray paint on canvas, was the last lot of Sotheby’s “Frieze Week” evening contemporary art sale. After competition between two telephone bidders, it was hammered down by the auctioneer Oliver Barker for 1 million pounds, more than three times the estimate and a new auction high for a work solely by the artist, according to Sotheby’s.

“Then we heard an alarm go off,” Morgan Long, the head of art investment at the London-based advisory firm the Fine Art Group, who was sitting in the front row of the room, said in an interview on Saturday. “Everyone turned round, and the picture had slipped through its frame.”

The painting, mounted on a wall close to a row of Sotheby’s staff members, had been shredded, or at least partially-shredded, by a remote-control mechanism on the back of the frame.

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“We’ve been Banksy-ed,” Alex Branczik, Sotheby’s head of contemporary art in Europe, said at a news conference afterward.

“I’ll be quite honest,” Mr. Branczik continued, “we have not experienced this situation in the past, where a painting is spontaneously shredded upon achieving a record for the artist.”

Mr. Branczik added that he was “not in on the ruse.”

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On Saturday afternoon Banksy posted a video on his Instagram account, recording the confusion at Sotheby’s auction, following a sequence purporting to show the artist hiding a shredder inside a gilt-wood picture frame. “A few years ago I secretly built a shredder into a painting,” Banksy wrote in the video. “In case it was ever put up for auction.”