By CATHY BOOTH

At 3 a.m. and beyond for three nights last week, late-night travelers near Atlanta came upon an astonishing new rock group in the lounge at the Ramada Renaissance. There, goofing and jamming together, were U2's lead singer Bono Vox and guitarist the Edge as well as Lou Reed, the grandfather of punk, Genesis Founder Peter Gabriel and New Orleans' own rock-'n'-soul kings, the Neville Brothers. Gyrating happily on the dance floor to the improvised mix of music was Joan Baez. This nightly ritual was the after-hours afterglow of the first-ever rock- 'n'-roll caravan for human rights, a six-city rolling tour of rockers, to focus U.S. attention on the victims of political persecution and torture around the world. Staged by Amnesty International to mark its 25th anniversary, the ''Conspiracy of Hope'' tour hopscotched across America by chartered 707 jet with the aforementioned lot, plus Sting and Canadian Rocker Bryan Adams, along with assorted one-night stand-ins including Bob Dylan and Bob Geldof. Very nice crew indeed, but really, aren't benefit concerts wearing a trifle thin? ''Everyone wanted us to go away after Live Aid,'' concedes Bono, an unquenchable Irishman. ''The music industry would be delighted to get back to packaging us like perfume commercials.'' But the pull of just one more won out. The tour was the idea of John Healey, 48, an ex-Franciscan monk and Peace Corps worker who heads the American office of Amnesty that goes after left- and right-wing oppressors, won the 1977 Nobel Peace Prize, but remains little known in the U.S. ''We're a household name in Europe. I want a grassroots- level recognition here too,'' Healey explains. At first he had trouble getting artists. Many were ''aided'' out. ''Six weeks ago I had just about decided it was easier to deal with dictators than rock 'n' rollers,'' said Healey with a grin. A few acts were quick to sign on, however. Sting, who has been on the road with a new band for the past 18 months, says only Amnesty could have dragged him out of bed to perform. Because of Bono's sympathies, U2 canceled two summer tour weeks to play for free. In the end, Healey had a suitably superstar lineup, snaring Ruben Blades, Jackson Browne, Miles Davis, Carlos Santana and Pete Townshend for spot appearances. Even Sean Penn and Madonna emceed in Los Angeles, and she promised to join Mia Farrow, Anjelica Huston and Meryl Streep at the New York-New Jersey tour finale in Giants Stadium. During the eleven-day journey, 114,000 concertgoers paid a hefty $36 each to simmer in what they hoped would be the best sounds of summer. Few were disappointed. Peter Gabriel gained legions of new fans with an electric delivery of his hit Sledgehammer as well as with Biko, dedicated to the South African martyr. In Atlanta, the Police played together for the first time in two years and took the crowd's breath away with Roxanne and Message in a Bottle. In Los Angeles, Live Aid's Bob Geldof, still blissfully unaware that he was about to be named a Knight of the British Empire, made a surprise appearance with Tom Petty & the Heartbreakers. (Don't call him Sir Bob, please. Because he is not British, he'll just be Mr. Geldof, Boomtown Rat, K.B.E.) But at almost every concert the fans belonged to U2, as Bono and The Edge ripped out New Year's Day and Sunday, Bloody Sunday. The girls threw lingerie, the guys waved Irish flags, and everybody screamed wildly when Bono lifted girls onstage, hugged them and left them in puddles of tears. The politics took a quieter tack. A slick ten-minute film delivered Amnesty's message, and the musicians talked about their involvement to the press. A.I. believes in the simple--as Gabriel calls it, ''almost ridiculous''--idea of getting members to write postcards and letters to governments on behalf of specific prisoners. Sting says he finds the work fun. ''As a pen pal, you get to write the leader of some repressive regime, basically making a nuisance of yourself. It's the rebelliousness of rock turned to good use at last!'' The message was apparently getting through. Kids leaving concerts were picking up handout postcards they had earlier tossed on the floor. By the end of the tour, Amnesty hopes to enlist thousands of new recruits for its write- in campaigns. At one stop Bono apologized. ''We are far better at getting Amnesty International's message across in music than press conferences. We're just good at very noisy rock 'n' roll.'' For those who came, whether for the concert or the cause, the noise was just right.