"The very worst that could happen would be to forget the Afghan freedom fighters . . . In the midst of our own liberty, we should not forget their lack of it," Vice President George Bush told an applauding audience of nearly 2,000 at the Kennedy Center Opera House.

Yesterday was an attempt to remember, as Afghanistan Day was observed around the nation and worldwide.

More than two years after Russian soldiers crossed the borders into Afghanistan--two days after Christmas 1979--Bush emphasized that the Soviet invasion and occupation of the small country cannot be allowed to be forgotten because it is no longer on the "front pages every day."

President Reagan observed Afghanistan Day, according to Bush, by sending a direct appeal to Soviet President Leonid I. Brezhnev, speaking of the Afghanistan occupation as "seriously poisoning the international environment" and calling for a "genuine and intensive search" for a peaceful resolution.

As the invited audience of government, diplomatic, church and ethnic organizations listened to the formal program in the ornate Opera House, others stood on the wet grass at the Sylvan Theater on the Washington Monument grounds, carrying handmade banners and placards calling for the more than 100,000 Soviet troops to leave Afghanistan.

The outdoor rally, sponsored by the Committee for a Free Afghanistan, drew several hundred people who heard speakers condemn the Soviet invasion and call for "Afghanistan Ahzod"--which means "Free Afghanistan."

Earlier, about 200 protesters marched near the Soviet Embassy and shouted "Down with the Soviet Union!" Police said both demonstrations were orderly.

Among the speakers at the Sylvan Theater were representatives from several ethnic organizations and also congressmen, including Rep. Larry McDonald (D-Ga.).

Holding up one end of a large banner calling for Afghan freedom was a 23-year-old who said he fled his homeland because of the Russian invasion.

"The first time, my people fighted," he said in stumbling English. "I fled to Pakistan. We had only empty hands--no guns."

Washington's observance of Afghanistan Day was one of more than a dozen yesterday around the country and the world. In New York, an estimated 1,500 people held a peaceful rally near the Soviet Mission to the United Nations, and in other American cities protesters numbered in the hundreds.

In New Delhi, Afghan refugees chanted "Death to Brezhnev" and burned a Soviet flag outside the Soviet Embassy. Anti-Soviet demonstrations were reported in Frankfurt, West Germany, and in Pakistan, which is home for nearly 2.5 million Afghans fleeing the war. In Afghanistan itself, about 200,000 people demonstrated at the U.S. embassy in a counterprotest.

At the Kennedy Center, the vice president held a miniature land mine in his palm before an audience that had undergone purse searches and passed through security metal detectors.

"What I have in my hand is a Soviet mine," Bush said. "It's called a 'butterfly' mine because it flutters softly to the ground," attracting children who think that the mines are toys. "The Soviets dropped thousands of these devices on Afghanistan."

Bush himself drew fire from rally organizers who complained he had sought the comfort of the Kennedy Center rather than appear on the muddy monument grounds. In the Opera House, the vice president spoke at the special Afghanistan Day program organized under the White House's aegis. The hour-long event included a folk dance by young Afghan refugees and a slide show narrated by actress Joan Fontaine, who recently visited the Afghan-Pakistani border.

What began as a travelogue of a small country with spectacular mountain scenery turned into scenes of the terror under which the Afghanistan people are living during the Soviet occupation. Three million Afghans--one in five of the country's population--have fled their homeland. The refugee problem is heart-rending, Fontaine said, as photographs of crowded tent cities appeared on the screen, because the Afghans have fled to neighboring countries too poor to offer much.

A French doctor who works with "Doctors Without Frontiers," an organization that has been sending medical personnel into Afghanistan surreptitiously despite Soviet disapproval, told of the maiming of children, who pick up the "butterfly" miniature mines and think that they have found a new toy.

"These are weapons of terror--disguised as children's toys," Dr. Claude Malhuret emphasized.

Malhuret said that last November three Afghan hospitals staffed by his organization were leveled by pinpoint Soviet fire. But he vowed, "We are in Afghanistan to stay."

Former secretary of state William P. Rogers, who headed the private-sector committee coordinating the Afghanistan Day observance in the United States, made the introductions at the Kennedy Center program and told a story that set the tone of the day and drew applause.

Rogers reported that, after one of the United Nations debates on the Afghanistan invasion, a United States diplomat asked a Russian colleague in private conversation to explain why the Soviets had done such a thing in the face of world condemnation.

"The world will get used to it," the Russian replied. After telling the story, Rogers added:

"We are here to prove to the Soviet Union the the world will not get used to it."