The New Year's Eve millennium fireworks show on the Mall will begin with a spectacular, 10-second countdown in which 10 pyrotechnic cascades will ascend the Washington Monument in 50-foot increments until the entire structure is illuminated at midnight, producers said yesterday.

Plans are to have President Clinton light a long fuse at the Lincoln Memorial that will race to the foot of the monument at 11:59:50 and start the countdown of the final seconds of the millennium.

"I can safely say without great exaggeration that it will be a once-in-a-lifetime sight," said George Stevens Jr., co-producer of the extravaganza on the Mall, which is expected to draw millions of spectators in person and on television.

Marsha Berry, Hillary Rodham Clinton's press secretary, who has been helping to coordinate events, said that the display is being called "a midnight moment" and that care is being taken not to damage the monument.

The countdown, which will be the first of two segments of the fireworks show, is being created by the country's most venerable fireworks company, Fireworks by Grucci, of Brookhaven, N.Y.

Grucci, a family-owned firm whose roots in the business go back to Italy in the 1850s, has done fireworks for the last five presidential inaugurations and was involved in the memorable displays at the centennials of the Statue of Liberty and the Brooklyn Bridge.

"It's a once-in-a-millennium opportunity," said Grucci's M. Philip Butler, who is producing the display. "It will never be allowed again, until the next millennium."

Butler said his technicians will be able to use the repair scaffolding that now covers the 555-foot monument to rig up a series of waterfall-like firework cascades every 50 feet on the monument's west face.

"It will be a very, very stunningly beautiful effect," Butler said. "It looks like white diamonds." Each will build upon the other, and the entire monument will be illuminated for about 20 seconds, he said. "At midnight there'll be 500 feet of waterfalls, of candle power I can't even guess."

The fuse will travel from the Lincoln Memorial, where the president is to be stationed during the evening's entertainment, the length of the Reflecting Pool to ignite the countdown.

"It's all new," Butler said. "This is special effects to a level that's never been attempted. Can you imagine if the damned fuse doesn't work or doesn't work properly? The whole world will be watching."

The countdown will be followed by the ignition of a 50-foot-high illuminated "2000" in front of the monument and pyrotechnic cascades of "split comets" behind it for 30 seconds.

Butler said there will be more entertainment for about 50 minutes and then a five-minute fireworks finale fired from two barges anchored in the Potomac behind the Lincoln Memorial.

Asked the show's cost, he replied: "You wouldn't ask Michelangelo how many buckets of paint he used on the Sistine Chapel. Let's just say it's a high six figures. It's not a standard fireworks show."

CAPTION: Workers erect the stage for the New Year's Eve celebration across from the Lincoln Memorial, where the fuse will begin.