On a search mission for guerrilla sympathizers, a dozen Salvadoran soldiers with M-16s crossed the border into Honduras last Nov. 16 and raided a refugee camp at La Virtud. They rounded up 15 of their countrymen to march them the four miles back to El Salvador. Foreign observers, witnessing the event, chased the abductors, snapped pictures and shouted: "The world is going to know what happened."

The troops smashed the cameras, confiscated the film, and turned their guns on the foreigners, some of whom bravely wrestled with the soldiers. In the confusion, the refugees managed to escape, and the Salvadoran invaders retreated. For one of the observers, Bianca Jagger, who had flown in to inspect refugee camps with visiting church and relief officials, the experience was transforming. "It is shocking," says the onetime playgirl, "to see unarmed civilians taken away to be killed, which is what the soldiers threatened."

For a decade Nicaraguan-born Bianca, 37, has participated in relief efforts for Central America's homeless. In 1973 she persuaded then husband Mick Jagger and the Rolling Stones to stage a benefit concert for earthquake victims in her native land. But since the refugee kidnap attempt, Bianca has become a political activist. She filed reports on the La Virtud incident with Amnesty International and has been lobbying on Capitol Hill. By her own account, Jagger discussed the upheaval in Central America with "at least 40 Congressmen," and she has instigated protest letters to President Reagan and Secretary of State Alexander Haig. She hotly disputes Haig's claim that the leftist regime in her homeland is supplying arms to Salvadoran guerrillas. "He's just blowing everything out of proportion," she claims. "The Nicaraguans have enough problems without sending aid to Salvadoran insurgents." Although she is a pacifist, Bianca apparently sympathizes with El Salvador's rebels. "Sometimes people have no alternative but to pick up arms," she says. Her lapel button sums up her views on U.S. policy: "Salvador is Spanish for Vietnam."

Bianca wasn't taken seriously last December when she first appeared at House Inter-American Affairs Subcommittee hearings. During her testimony a crowd of legislative aides gathered, as one put it, "to gawk at the woman who married a rock 'n' roll star." Conservative in dress and demeanor, Bianca impressed her skeptical audience. "I was pleasantly surprised," said Rep. Robert Garcia of New York. "Her testimony was poignant and she proved to be very intelligent. She was a damned good advocate for the refugee problem."

The daughter of a wealthy import-export merchant, Bianca admits that as a child "I never washed a dish, boiled an egg or cleaned." By the age of 19, she had joined the jet set. Her eight-year marriage to Mick, which ended in 1979, left her with a daughter, Jade, now 10, thousands of headlines and a frivolous reputation. To underscore her seriousness these days, Bianca declines to discuss Jagger, and she's rarely seen at discos. She is convinced that her image as a celebrity hurts her political credibility. "Sometimes people have you in a little box and they feel lost when you move out of that box," she says, "so they react with skepticism." Last week she returned to the turbulent region on another tour to document the plight of refugees. She insists that the prospect of violence will not deter her. "When the soldiers pointed guns at us, sure I was scared," she says. "But I had to go back. I cannot remain detached."