La Ronda, the grand Gothic castle that presided over a Bryn Mawr neighborhood for eight decades, is all but gone.

Five minutes after a township demolition permit allowed work to start yesterday morning, the long arm of a yellow excavator took the first bite of the mansion's facade, sending shards of glass, wood, and stucco crashing to the ground.

The machine's metal jaws chewed through walls and pieces of the Spanish-style roof. By day's end, three-quarters of the building was rubble. All that was left of the 51-room house were the three-story tower and a piece of a library.

The demolition contractor said it would be weeks before all remnants of the building were removed. "As you can see, it moves fast," said Keith Brubacher, who owns Brubacher Excavating Inc.

The building's fate attracted the attention of preservationists and others who fought the planned demolition of the house designed by Addison Mizner at 1030 Mount Pleasant Rd.

Throughout the day, a stream of onlookers drove or walked by La Ronda - including Gladwyne Elementary School children who shouted "Save La Ronda" out the window of their yellow bus. At dusk, vehicles slowed as occupants snapped photos.

"I'm sad," said Eileen Bloom, 63, a Lower Merion resident who came to see the spectacle. "I've lived here my whole life. It's a piece of history, and it's going, and this is what's happening to the Main Line."

The mansion was purchased in March for $6 million by Joseph and Sharon Kestenbaum of Penn Valley behind a pair of corporate identities. Plans were filed with the township to tear it down and replace it with a new house.

Ross Mitchell, vice president of the Lower Merion Township Historical Society, said yesterday he was shocked that a deal could not be struck to save the mansion. "He could have built his house anywhere," Mitchell said.

Kestenbaum's spokesman, Jeff Jubelirer, responded, "Mr. Mitchell could have purchased the home or the property and done whatever he wanted. He had from March till Sept. 18 to make an offer and raise the money.

"He didn't execute, so Mr. Kestenbaum decided to do what he wanted to do in building his family's home."

Township Commissioners President Bruce D. Reed said yesterday that he was devastated that the house could not be saved. "It's a tremendous loss, as many of us said it would be from the outset, and I think it will be viewed as such for years to come."

At one point yesterday, Angela Kania, the wife of La Ronda's previous owner, Arthur Kania, drove by and appeared upset at the wreckage of her former home: "This is a funeral. Can't you see I'm wearing black?"

Linda Kania Graupner and Liz Kania cried and embraced in the street in front of the mansion. The women talked of happy weddings and rehearsal dinners there.

Doug Mellor, who lives next door, looked crushed.

"I'm despondent," Mellor said. "I want to cry."

The mechanical excavator, moving on treads like those of a bulldozer, used the rubble to build a platform from which to attack the next wall or roof.

Several times the operator punctured the mansion's supports with a metal I-beam taken from the house, prompting preservationist Lori Salganicoff to comment: "Do you see what they're doing?

"They're using a piece of the building to destroy itself. This is surreal."