NEW YORK (AP) _ This time around, there'll be no President Reagan, no fireworks, no Elvis impersonators, no jazzercise dancers. Instead the Statue of Liberty's 100th birthday will be observed sedately, with speeches, medals and classical music.

''We're not trying to outdo Liberty Weekend, because it's impossible to do that,'' said Interior Department spokesman Alan Levitt, referring to the glitzy Fourth of July weekend extravaganza. ''We'll have a low-key celebration.''

The 100th anniversary of the statue's dedication, which took place Oct. 28, 1886, will be marked by three main events: an Oct. 27 Ellis Island ceremony honoring 80 Americans of diverse ethnic backgrounds; a rededication ceremony the next day on Liberty Island, and that night, at Lincoln Center, ''Liberty's Centennial Gala: The All-Star Finale.'' ''We're calling it that because people keep asking, 'Is this the end?''' said organizer Nancy Sureck.

The Interior Department is planning the Liberty Island ceremony. The other events are the work of the Statue of Liberty-Ellis Island Foundation, a quasi- public group which has raised money to fix up the statue and the closed immigration hall nearby.

The Oct. 28 rededication will feature speeches by Interior Secretary Donald Hodel, Treasury Secretary James Baker and Arman Avedisian, the man Hodel chose to replace Lee Iacocca as chairman of the government's Statue of Liberty-Ellis Island Commission.

Among the invited guests are Iacocca, Gov. Mario Cuomo, Mayor Edward I. Koch and a delegation from France, which gave the statue to the United States. There also will be room for the first 3,000 members of the public who ride the ferry to the island that morning, Levitt said.

The statue will be designated as a ''World Heritage Site,'' joining such wonders as the Taj Mahal, the Great Pyramids and the Grand Canyon.

The medals to be awarded Oct. 27 are an attempt to placate members of several ethnic groups who protested their exclusion from the Medal of Liberty ceremony July 3 on Governors Island.

President Reagan presented the medals to 12 naturalized citizens, none of whom was of Irish, Italian, French or Swedish extraction.

''It will remind those groups that they are as much a part of building America as the two or three ethnic groups that received 12 medals,'' said Dan Klores, a spokesman for the National Ethnic Coalition of Organizations.

The 80 Ellis Island Medal of Honor recipients include boxer Muhammad Ali, singer John Denver, developer Donald Trump, former first lady Jacqueline Kennedy Onassis, baseball great Joe DiMaggio and broadcaster Walter Cronkite.

The group, which includes fewer than 20 women, was selected from 15,000 nominees.

The musical salute at Avery Fisher Hall will feature the world premiere of ''On Freedom's Ground: An American Cantata'' by contemporary classical composer William Schuman, with a text by poet Richard Wilbur. The piece will be performed by the New York Philharmonic, led by Zubin Mehta, and a 200- member chorus.

Later, Julio Iglesias, Charles Aznavour and Placido Domingo will sing individually and team up for the finale.