President Carter announced last night the the National Christmas Tree, a nationwide symbol of the promise of Christmas, would remain unlighted this year until American hostages in Iran are set free.

In a brief but dramatic gesture at the end of an otherwise routine seasonal ceremony, the president sent his daughter Amy forward to throw the switch that normally lights the 30-foot-high blue spruce and the 50 smaller Christmas trees nearby representing the 50 states.

But only the star atop the big tree lit up, together with the tiny blue lights on the smaller trees.

"Amy has lit 50 trees -- one for each American hostage," the president told the stunned crowd of 7,500 gathered on the Ellipse just south of the White House. "We will turn on the rest of the lights when the hostages come home."

After a moment of silence, the audience burst into applause.

"I thought it was super," said Mary Jane Mastorovich, 33, of Falls Church. "It was a warm effort to signify the hostages' plight."

"It makes it more meaningful to see a darkened tree," said Steve Bryant, 35, of Reston, who had come for the tree-lighting ceremony with his two sons, aged 4 and 9. "It symbolizes the suffering of the hostages."

There had been no warning that the tree-lighting ceremony would be anything other than a traditional Washigton Christmas event, at which tourists and federal workers turn out to hear carols, see the lights and watch reindeer browsing near the White House grounds.

In his brief speech the presdient asked the audience on the Ellipse and those watcing television to join in silent prayer that "American hostages will come back safe and come home soon."

He said that while Christmas is "a time of love, warmth, friendship, light and family," everyone this Christmas will not share these feeling "because there are 50 Americans in Iran who don't have these things."

He said the year had seen much in the way of hope for the world, including the signing of the Eygptian-Israeli peace treaty and the promise of peace in Zimbabwe-Rhodesia.

But he said much pain remains, including the suffering in Cambodia "where children are too weak and hungry to cry" and the crisis in Iran, which he termed the product of "a misguided application of belief in God."

"We do have suffering . . . but in its midst . . . we can remember what Christmas is: a time of love, a time of family and a time of peace," the president said.

Then the president announced: "Now Amy will throw the switch."

She did so; the president made his surprise announcement, and shortly afterward returned with Amy and Mrs. Carter to the White House.

National Park Service officials said the president's gesture marks the first time in 56 years that the nation's Christmas tree has remained dark during the Christmas season.

A giant evergreen has been decorated with lights near the White House or on the Ellipse since the administration of Calvin Coolidge in 1923.

The current tree was trimmed earlier in the week with large silver balls and light bulbs and other aspects of the tree-lighting ceremony remained as planned, from the traditional reindeer to carols played by the Marine Band and sung this year by the 115-voice Neshaminy Maple Point Concert Choir from Langhorne, Pa.