What do stars do on their night off? They shine anyway.

At least that was the case as Petula Clark, David Cassidy and the cast of Blood Brothers gave up their free Monday night to put on a musical review at Bimbo's to benefit Broadway Cares/Equity Fights AIDS. The cause was noble, the mood festive and the singing as good as it gets.

The beginning was magical. Lights dimmed, the din was hushed and Clark's inimitable voice broke the silence with an improvisation ending in the single word "Downtown." The crowd went crazy.

She then appeared atop a grand piano singing "Somewhere," by the Moody Blues. Looking at once earthy and ravishing, her voice miraculously fresh, the pop diva quickly moved on to Tony Hatch's "Don't Sleep in the Subway" and a generous set of a dozen British songs by everyone from Elton John to Andrew Lloyd Webber.

The voice was fragile at first, the phrasing sensual from the start. By the time she warmed up in "Subway," the familiar Pet sound was sweeter than memory might suggest. Here was the voice that defined the urbane, sexier side of the ‘60's British Invasion. The public gave up a lot when it allowed the divorce of pop from rock and roll. France kept its Mireille Matthieu, Italy its Mina and Milva, England its Pet Clark. Barbra Streisand still nurtures the pop tradition on this side of the Atlantic. But these are the exceptions to a rule that keeps songs such as Hatch's from reaching the mass audience they once enjoyed. "Strangers Once Again," a touching new tune from Clark's next album, hinted that the future may be brighter. The best news of all was Clark's singing itself, however. Her stage banter was winning too.

"I knew Elton long before he had hair," she quipped before launching into his
" Crocodile Rock" and other hits from the ‘70s, "before he got married and confused millions." She continued her "bouquet from the U.K." with "Don't Cry for Me, Argentina," which made stunning use of the trademark Clark grace notes and brought a flavor all its own to the Lloyd Webber hit.

"My Love" had the audience giddy with pleasure. "Downtown" had it singing back the chorus. "All my life I have waited to play Bimbo's," joked this woman who has commanded the stage from the Copacabana to the Olympia, from Broadway to the West End. In truth, there can have been but few happier performances in her illustrious career than this intimate, exuberant concert in San Francisco Monday night.

Cassidy had a tough act to follow, but he, too, knows a thing or two about bringing a crowd to a frenzy. He captured the spirit of Bimbo's very well: Incredibly cool, totally retro, shamelessly entertaining, Cassidy put together a tribute to the late Bobby Darin that fit the place as naturally as it fit his attractive voice.

"Dream Lover" was delicious, "Up a Lazy River" even better. Composers as varied as Charles Trenet and Kurt Weil brought out the best in this singer, who would do well to record his own cover of "Mack the Knife." A crooner, a belter and a pretty sexy guy, Cassidy managed to pay homage to his idol at the same time as he reminded us of his own considerable magnetism. The Blood Brother musicians, with particularly excellent saxophones and drums, also donated their time for the AIDS benefit along with two attractive sets of backup vocal trios from the cast: Perry Ojeda, Leslie Ann Hendricks, Mark McGrath, Marcy D. Nezza, Yvette Lawrence and Walter Hudson.

The six young singers joined the two stars for the finale, which began with Clark and Cassidy each singing one of the other's hits. Clark had a ball with "I Think I Love You," although bubble gum is clearly not her strong suit. Cassidy, for his part, really soared in "I Know a Place," which brought the pair together before a rousing finale of "Tell Me It's Not True" from Blood Brothers. The cheers at the end were long, loud and deserved.