The band's just warming up and already the crowd's on its feet and rocking in the aisles.

A pulsating line dance begins snaking through the sanctuary. Some parents hoist toddlers atop their shoulders and join in while others in the evening's crowd of more than 200 stand in place, clapping to the amplified beat.

This isn't exactly your father's Conservative synagogue.

While many Reform synagogues have had instrumental music at their services for years, the Conservative movement traditionally refrained on halachic grounds.

Now, the Family Rock and Roll Shabbat at Oakland's Temple Beth Abraham is part of a growing trend rumbling through Conservative congregations on both coasts.

Throughout the Bay Area, Conservative synagogues are increasingly celebrating services to the tune of instrumental and even pop accompaniment.

And when music is played, rabbis report that attendance is up — sometimes dramatically.

Besides the occasional events at Temple Beth Abraham, musical Shabbats also have sprung up at Tiburon's Congregation Kol Shofar, San Francisco's Congregation Ner Tamid, Petaluma's Congregation B'nai Israel and, on a limited basis, Walnut Creek's Congregation B'nai Shalom in Walnut Creek. The subject has been under review at Peninsula Sinai Congregation in Foster City.

Beth Abraham fired up its band, under the direction of Rabbi Mark Bloom, a year ago. It has played five times since.

Along with traditional religious music, the band played a medley of peace songs at its debut.

"It went from 'Shalom Rav' to 'Peace Train,'" said Bloom, who admits to some jittery moments before that first gig.

"I was really worried about how the elderly would respond," said the rabbi, who plays amplified acoustical guitar, sings and leads the band. Composed of a line-up of homegrown talent, the multigenerational group sings and plays instruments ranging from electric guitar to sax.

The band doesn't play at Saturday morning services because, Bloom explains, "we want a more traditional feel to that."

But despite his concerns about congregational acceptance, Bloom quickly discovered that he had a hit on his hands.

Attendance at Friday night services surged from about 40 people to more than 200. And, to Bloom's relief and delight, "the elderly folks love it. They think it's the greatest thing you've ever seen."

Just ask 80-year-old Ellen Bercovich of Walnut Creek.

"If you think I'm offended, just the opposite," says Bercovich, who has been a member of the congregation since childhood.

While describing the electrified music as "wonderful fun," she also wonders what reaction the synagogue's founding fathers would have.

"God knows," she said with a chuckle.

Across the bay at Kol Shofar, Rabbi Lavey Derby readily acknowledges that "not everybody loves" his synagogue's recently introduced Shir Chadash (A New Song) service. Held monthly, the Friday night services are celebrated to the musical accompaniment of a guitar, drums or dumbek, a traditional Middle Eastern drum. Local musician Achi Ben-Shalom normally plays.

Before initiating the new service, congregants discussed the pros and cons, and had a few experimental programs.

"We all agreed that music enhances, sometimes greatly enhances, the spiritual effect of prayer and can enliven worship," Derby said.

He noted that attendance at musical services was "dramatically up," pulling in perhaps 130 more congregants than usual.

Not everyone is a fan, however. "We have people in the community who won't come to that service," said the rabbi.

One critic reportedly complained that the event felt too much like a service and, Derby said, "we've had a person say it felt like a sing-down."

He considers the new service part of a smorgasbord of offerings intended to appeal to different tastes in worship.

While many Conservative congregations, particularly on the East Coast, have celebrated services with organ music for decades, the movement previously shied away from other instrumental music on Shabbat.

The reason, rabbis explain, deals with the prohibition against working on Shabbat and with mourning for the destruction of the Temple, where instruments were played.

"Halachically, it's not that you can't play musical instruments," Bloom said. "It's that you can't carry them or fix them."

Such restrictions were "gray anyway," he said, noting that many Conservative synagogues already have microphones and sound systems.

Rabbi Joel Meyers, executive vice president of the movement's Rabbinical Assembly in New York, agreed that the issue is open to interpretation by individual rabbis. "The difficult part would be how flexible to be," said Meyers, who nonetheless doesn't think instrumental music is a widespread trend at Conservative synagogues.

A check of local Conservative congregations revealed a variety of approaches — and opinions.

San Francisco's Congregation B'nai Emunah had organ music at services until the death of its organist about a decade ago. The congregation now sings without accompaniment.

Congregants "think it would stifle their own participation and it would become sort of a concert," Rabbi Ted Alexander explained.

At San Francisco's Congregation Beth Sholom, members sing unaccompanied at services — and like it that way, says Rabbi Alan Lew.

"We have never wanted anything to detract from participation of the entire congregation."

Friday night services can draw 100 to 300 people, he says, and "it really proves to me that you don't need instruments to have a powerful musical experience."

For Lew, the vocal tradition at Beth Sholom speaks to the heart of Shabbat services.

"Shabbat is supposed to be about not having rock and roll," he said. "Shabbat is about not creating, not adding to what's there."

Though a self-described jazz fanatic, Lew said, "I do feel that rock and roll and other instrumental music do bring the world — that Shabbat is supposed to be a release from — into the synagogue."

Asked if Berkeley's Congregation Netivot Shalom had instrumental accompaniment, past President Debby Graudenz replied: "Absolutely not. We're in some ways a very traditional synagogue."

And at Temple Beth Sholom in San Leandro, Rabbi Harry Manhoff said his congregation is under the musical guidance of Cantor Linda Hirschhorn, a member of the internationally known singing group Vocolot. "It kind of would be strange for us to go to musical instruments when we have such a talent."

But at Ner Tamid, Rabbi Moshe Levin said instrumental music "does really enable people to participate and to feel uplifted."

The advent of instrumental music at Conservative synagogues isn't simply a West Coast phenomenon. Congregation B'nai Jeshurun in New York, formerly Conservative but now independent, introduced musical Shabbats back in the mid-1980s. Drawing heavily on Sephardic melodies as well as Chassidic niggunim (wordless melodies), the services typically attract a whopping 1,500 to 2,000 participants, who often join hands and dance in the aisles.

Beth Abraham congregant and band member Chester Regen has wrestled a bit with the issue religiously. "In the wintertime [with its shorter days and earlier candle-lighting times], it's kind of cheating," said the 44-year-old Oakland resident, who plays clarinet and alto sax. On the other hand, he noted, "it's important for Shabbat to be enjoyable to children."

The band was created after fellow congregant and vocalist Jeanne Korn approached Bloom about starting a musical Shabbat similar to those held at nearby Reform Temple Sinai.

Bloom bought the idea and the result, she said, has been "metamorphic.

"I just think music gives a different kind of voice to prayer and people can relate to it."

As proof, she points to attendance at those Shabbat services. "There will be people here from 2 to 85," she says, "and everyone is grooving to the spirit."

The band's repertoire is largely traditional songs and chants, such as "L'Cha Dodi" and the Barchu, set to contemporary music.

The music "really moves people on a really deep spiritual level," said Bloom. "Prayers speak to our minds and even our hearts, but music speaks to our soul."

It also speaks to toddlers, many of whom dance in the aisles. One small boy brings his toy guitar and strums along to the band up on the bimah.

"We have a 2-year-old so it's a treat to be able to bring her," says Josie Levi of Oakland. "I find myself humming the tunes to the different prayers a week after the rock and roll Shabbat."

Longtime members Arthur and Rosalie Beren, 78 and 77 respectively, are not big rock and roll enthusiasts but they are huge supporters of their synagogue's new band — and the effect it's had on the congregation.

"Whoever thought that davening would be to music, rock music?" asked Arthur Beren, a congregant for more than 50 years. "It's an electrifying experience to see the rabbi up on the bimah surrounded by the chorus and some instruments."

His wife agrees. "If you could have seen how happy everyone was, the young people and their children, it was worthwhile."

Still, she prefers some moderation. Rock services at shul, she said, should be "a once-in-a-while thing."