An anniversary is being observed in front of the White House today.

The Quaker Vigil for Peace will complete one year of protest against the Vietnam war -- one year in the rain, sun and snow; one year of commitment and hardship; one year of being stared at by tourists and, occasionally (in recent weeks) being arrested by police.

On June 2 of last year, a group of Quakers stationed themselves on the sidewalk in front of the White House, declaring their opposition to the war in impassioned rhetoric now a familiar staple of the peace movement.

It was all so familiar, in fact, that for a long time, no one paid much attention to the somewhat ragged band. They became sidewalk fixtures, with knapsacks and sleeping bags, peanut butter sandwiches and the sign saying "Give peace a chance."

But they are noticed now, because they have endured.

THERE MAY be only a few lounging by the fence at a given moment in a day. But later, there may be as many as 100. And although many have only maintained the vigil briefly, there are others who have been there, off and on, for the entire year.

Yesterday at noon, Seeds DiMarcos, 25, who described herself as a "street person," said she had been there at the beginning and would be there for today's anniversary, and beyond.

Sitting cross-legged on the sidewalk wrapped in a blanket and reading a dog-eared paperback, Miss DiMarcos told of her year of protest. Two men in business suits stopped and stared at her, their faces contemptuous. She did not notice them.

"I think we've been going through as much torture as the Viet Cong," she said. "I've sat in the rain all night, as I'm sure they do, praying that the sun would come in the morning and dry me out. I've been afraid to go to sleep because some cop might hassle me.

"I haven't had much to eat sometimes. In a sense, the only difference between me and the Viet Cong is that I don't have a gun in my hand."

While Miss DiMarcos compared herself with the Viet Cong, a girl in pink tights, wearing a cap adorned with an "impeach Nixon" button, danced on the sidewalk, to the surprise of passersby.

THE DANCER is 15. She has been with the vigil for a short time. She had been coming with her mother for about a month. She wants to be a dancer, she said, and when "Nixon comes in tonight (from abroad) I'm going to do a dance of welcome."

The girl's mother, who is 46, gave her first name as Ellen. She did not dare reveal her last name, she explained, because her husband works for the District government.

"I've been coming here with food for these people for about a month," she said. "I live in Prince Georges County and I used to walk by on the other side of the street and see those people and think that maybe I should join them. Then I'd dismiss the idea. But then one day I said 'why not?' What else is there?"

Brian Harrison, 26, of Boston, is another who was there off and on through the year. Only in the past few weeks, he said, have the police begun "hassling" demonstrators. The demonstrators were told they could no longer sleep on the sidewalk overnight. (Many of them now sleep in Lafayette Park across the street.) A few have been arrested for disorderly conduct.

Harrison said he thinks an order to crack down on the protesters came from the White House. "I'm sure Nixon has seen us out here and he wants to get rid of us."

HARRISON SAID foreign tourists are particularly friendly to the demonstrators and that there has been relatively little trouble with passersby.

"On Friday nights, sometimes," he said, "we get people going by in cars who yell at us and give us a hard time. Otherwise, people have been pretty good."

The vigil's best-known supporters are ex-Beatle John Lennon and his wife, Yoko Ono, who are scheduled to join a "walking marathon" today outside the White House protesting alleged police harassment of the demonstrators.

Several court cases involving the "harassment" are pending.

And yesterday, about noon, a florist's truck stopped at the curb near the demonstrators, delivering flowers from the Lennons. It was the third consecutive week John and Yoko had sent flowers to the vigil-keepers.

A card with the flowers, saying: "Peace and Love."