Nancy Miller, 16, a junior at Parkdale High School in Riverdale, said she had never been to a demonstration before yesterday. Neither had Frank Bourne, 62, a silver-haired retired U.S. foreign service officer and Air Force veteran.

But they both carried banners yesterday at Lafayette Square across from the White House among a crowd of several hundred persons at the spot they called "ground zero," the point where the first nuclear bomb might hit in the event of the unthinkable: nuclear war.

Miller and Bourne were among thousands who participated in the kickoff of Ground Zero week, a nonpartisan nationwide project to educate the American public about the possibility of nuclear war and the ways to avoid it.

Young and old, veteran demonstrators and novices, teachers and students, parents and children were all among those who planned to gather here and in hundreds of cities, towns, schools and campuses across America in a movement described by organizers as the largest protest drive in a decade. Films, church services, seminars and "town meetings" are planned throughout the week.

"Welcome to the finest and noblest cause that mankind has ever known," said Roger C. Molander, the founder of Ground Zero, as he greeted a Lafayette Square crowd in which some were barefoot and some wore business suits. "It would take only 30 minutes to end all of evolution, and all that God has created . . . The greatest challenge of the human community" is to stop it.

Earlier, at a special sermon at Washington Cathedral, Molander outlined a long-term strategy for Ground Zero, in which the movement would work to elect congressmen this fall and a president in 1984 based on candidates' positions on the nuclear issue. "Then," he said, "starting in 1985, we can come forward as a nation with a clear position, and deal with the Soviet Union" toward disarmament.

Molander, a former arms control specialist with the National Security Council, who founded Ground Zero some 10 months ago, described himself as formerly a member of a "technical priesthood" which believes that technical experts can achieve peace through processes like the SALT talks.

Now, he said, he believes a grass-roots movement of Americans must force their leaders to negotiate disarmament with the Soviets, despite American fears of Soviet aggression. The Soviets, he said, "are the all-time champion human rights violators but we can't wait for them to become a democracy" before addressing the nuclear issue.

As she held up a "Ground Zero" banner and tied a "Ban the Bomb" T-shirt around her neck, Nancy Miller said she was drawn to the demonstration by the recent realization that "we have enough bombs to blow up the whole world 19 times. It's scary."

Miller and her classmate, Lisa Speelman, 17, of Lanham, said they were moved to attend because they have been discussing nuclear issues in their foreign policy and politics class at their school. Miller also said she planned to stay active in the cause. "The people here are wonderful," she added. "They really care."

Carrying an American Veteran's Committee banner, Bourne said that after more than 20 years of interest in nuclear disarmament he was encouraged to come to Ground Zero because "I have a strong feeling that now is the time to express ourselves, because there is a feeling lately that it might make a difference."

Ross and Mary Jo Muir drove about 40 minutes from Woodbridge to hear Molander's sermon at the Washington Cathedral and said it appealed to them on both moral and practical grounds because it did not call for unilateral disarmament. "Unilateral disarmament is out of the question," Ross Muir said, "but some day we are going to have to really sit down and talk to the Russians."

But Norman Wilcox, a white-haired Norfolk banker visiting the cathedral, described Molander's message as "dangerous" because, he said, America must deal firmly with Soviet aggression and must be willing to threaten force, as the country did during the Cuban missile crisis in 1962.

Wilcox's daughter Jane, 21, a senior at Mount Vernon College, walked out during Molander's sermon because, she said, "Politics does not belong in the church."

Church groups have been active in the movement, however, with the Council of Churches of Greater Washington, representing about 550 churches, endorsing Ground Zero, according to the Rev. Ernest Gibson, the council's director.