On Wednesday night, while Mick Jagger undulated his thin, frenzied body on the stage of the Capital Centre before 19,000 fans, his ex-wife, Bianca, spent the evening in her room at the Fairfax Hotel, going over notes on Salvadoran refugees.

She is no longer a fan.

Bianca Jagger has come to Washington to talk about Salvadoran refugees, some of whom she saw bound by their thumbs and led off at gunpoint from a refugee camp in Honduras.

In the chill afternoon light, she sits sober-faced in a seminar room at the Institute for Policy Studies to tell her story. Robert White, the former U.S. ambassador to El Salvador, and State Department human rights expert George Lister have come to hear her. Isabel Letelier, a fellow at the Institute, introduces her:

"Bianca Jagger was born in Nicaragua. She studied at the Institute for Political Science in Paris. She has a daughter, and in 1973 she went to Nicaragua to raise money for relief for earthquake victims . . ."

The introduction leaves a gap. There's no mention of the nights and early mornings at Studio 54 or the time designer Halston threw her a birthday party there. She trotted in on a white horse to cut her cake while guests like Mikhail Baryshnikov looked on.

There is no mention of the marriage to Mick Jagger in St. Tropez in 1971 or the divorce proceedings eight years later when she called him a "liar" and a "tax dodger" in pre-trial hearings and filed papers in Los Angeles claiming their married life had been a "nomadic journey from one place to another in his quest to avoid taxes."

The walking stick that became her trademark during those years of jetsetting and partying is gone. But the polish that it reflected is still there. Her clothing is understated but elegant -- a tailored black-and-white houndstooth pant suit with maroon scarf tied at the neck of her white blouse. On her feet are black patent-leather high-heeled pumps. Nearsighted and in need of glasses, she squints occasionally when she looks out into a crowd.

The excesses of a life style high on fame and frivolity have not faded the beauty that distinguished her. She is 31, she says -- not five years older, as has been reported -- and produces a passport to prove it.

The coffee-brown skin is smooth and flawless. The high cheekbones nearly burst out of the taut square face. Her mouth is extraordinarily wide and full. It bears a remarkable resemblance to the mouth of the man from whom she derives her own fame -- her ex-husband, Mick Jagger.

When Letelier spoke of Bianca Jagger's efforts on behalf of victims of the 1973 Nicaraguan earthquake, she said, "Together, with her former husband, they raised funds . . ."

Raising a ghost but never speaking his name.

Jagger hadn't even known that her former husband was in town, staying only a mile away from her. "I read it in the paper," she says the day after her IPS seminar. "It's very ironic." She grins and leans back on a couch in her room.

Spread before her on the coffee table are stacks of black-and-white photos of scenes from the Salvadoran camp in La Virtud in Honduras. Along with these, she carts around notes and statistics on refugees from a civil war that has claimed more than 10,000 lives in El Salvador in the last nine months, according to another speaker at the seminar. Over the past two days, Jagger has talked to human rights activists and members of Congress such as Barbara Mikulski (D-Md.) and Patricia Schroeder (D-Colo.). Last month she gave a speech at Harvard and last night she was scheduled to speak at a dinner sponsored by a Catholic relief organization.

She is unpretentious and armed with information about Central America. The facts she spouts have won her some kudos from listeners. "I was surprised that she was so well-versed," said an aide to one member of Congress, "in anything."

Jagger is the daughter of a wealthy Nicaraguan, now retired from the import-export business. "I grew up wealthy in that society," she says. "That doesn't mean I can't see both sides. When my mother was divorced, she had to work. I wouldn't say I saw poverty. But the discrepancy in that society is so great that it's not hard to see."

She became interested in the plight of Salvadoran refugees while visiting her father in Nicaragua in September. Her first trip to the Honduran camps was made in early fall with some French journalists who had invited her to accompany them. She made her second trip on Nov. 16, traveling by plane and then jeep to La Virtud, the site of the refugee camp, about six kilometers from the Honduran border. She tells the story over and over again during her stay, in clinical detail:

"At 1:15 we were having lunch with Doctors Without Borders a group of French doctors administering care to the refugees . It was across the street from the Honduran command post. A relief worker came rushing in. He said, 'Men in civilian clothes are taking the refugees.' So we all rushed into a jeep and drove to the camp. We could see them being taken away." Jagger says they saw about 20 men in civilian clothing carrying M16s.

From a bridge, Jagger says, they could see refugees being led through a ravine. "We saw two men wrestling. We started screaming, 'We are the international press. The world will know what you're doing. You're taking innocent and defenseless people.' " Jagger and her party -- including Russell Davenport from the relief agency Oxfam, and Robert Brauer, an aide to Rep. Ronald Dellums (D-Calif.) -- left their jeep and ran after the group of men, taking pictures as they went.

She produces photos of men with their hands tied behind their backs. There are men with rifles beside them, but no one is identifiable and some of the photos are fuzzy.

"One woman refugee was carrying a child in each arm. She was kicked twice. Another woman was pregnant," says Jagger. ". . . As we got in close behind them, the ORDEN members the paramilitary rightist death squad that Jagger said the refugees identified as the gunmen went ahead of the refugees. The refugees took that opportunity to run back to us. The ORDEN members felt we were too close. We could identify them."

Jagger and her party returned with the refugees to the camp only to find that Salvadoran soldiers were hustling away more refugees there. Again they screamed at them. "This time we also said we had staff from senators' offices." She produces more photos. "You see how tying the thumbs behind the back makes the refugees bend over slightly?" she says. "Look at the resignation."

"We got back all these people," says Jagger. "But it's really quite amazing. This can happen anytime. They could be shot and the Salvadoran military could say, 'We found these people involved in guerrilla activity.' "

That night, they all slept on blankets at the campground to guard the refugees from further raids. Jagger says relocation of these camps will not solve such illegal raids by Salvadoran military and paramilitary groups. "What we're asking for is the following," she says. "To ensure the neutrality of Honduran military and to put an end to the continuing collaboration between Salvadorans and Hondurans."

"You know," she muses, "the one thing you remember is the rifle. We all felt endangered -- the relief workers, the refugees, everyone. That is what is so frightening."

She pauses and takes a long drag on a Marlboro. So different, all this, from the partying she is known for.

"It has limited the things that I can do publicly," she says. "I can't be involved in an issue like this and go to parties. It's the price you pay. It's not a matter of what I like. It's a matter of what's appropriate."

Her efforts for the refugees thus far have been informational. She has not, for instance, asked her former husband to repeat the benefit performance in 1973 that raised $280,000 for victims of the Nicaraguan earthquake. "I would love it if he would do a concert," she says. "I never asked him. If I felt he could do it, I would ask him."

She also did not attend any of the Capital Centre concerts this week. "Nooooo," she breathes in a husky voice, almost admonishing, when asked. "I saw him in New York . . . It is the best show around, no matter what I think of him," she says.

She met Mick Jagger in Paris, where she was a student. "Yes, I thought he was very famous then," she says with a smile. "We got married six months later." She shrugs. "It happens. You're young. You meet someone. You fall in love. You get married."

She had her daughter, Jade, at 21. "It was young," she acknowledges, wrapping her arms around her for warmth in her chilly hotel room. "I'm glad I had a child."

When Bianca and Mick do talk, it is about Jade, she says.

Now, she wants to be on her own. "As a woman, the most important thing for me is to be an independent person. I want to make my own life."

Why didn't she drop the Jagger, then, and return to Bianca Perez Mora Macias?

"It's too difficult," she says, shaking her head. "If you're a well-known person, it's difficult to change your name."

Then how can she ever leave the Jagger umbrella of fame that rests comfortably over her head and be really independent?

She takes a bite of a strawberry from the room-service tray. "Some magazines call me Bianca," she says. "Simply call me Bianca."