One of the region's most prominent outdoor concert series begins its summer run tomorrow night with a performance by opera singers Angela Horn and Tom Barrett. The series continues over the next six weeks with shows by artists who include Prince Charles's former harpist and a singer-songwriter from the "Ally McBeal" television show.

Over the previous six years, the River Concert Series at St. Mary's College of Maryland has grown in stature and popularity. In 1999, about 5,000 people turned out for the free series along the banks of the St. Mary's River; last year, about 35,000 people attended the performances, said college spokesman Marc Apter.

"It has grown dramatically," he said. "You think, well, it's a classical concert, so you'd see older people. But if you look at the crowd, it's everyone from little kids to high schoolers to college kids on dates to families to older people. . . . The way people socialize in the wintertime in St. Mary's County is they go to Wal-Mart. In the summertime they come to the River Concert Series."

This year, the series encompasses seven Friday night concerts featuring the Chesapeake Orchestra, led by music director Jeffrey Silberschlag, with guest performers each week. The series starts with Horn, a mezzo-soprano who sang the title role of "Carmen" with the New York City Opera and was named its Outstanding Artist of the Year in 1998; and Barrett, a baritone who has performed with opera companies in the United States and Europe.

Silberschlag said the series began in 1999 in response to community interest in having more cultural events and a place for people to gather on warm summer nights. He said he and the orchestra try to have fun with the music by including acts that will appeal to audiences beyond classical music buffs. For instance, the July 1 show will include Morton Gould's 1995 composition "Hosedown -- A Firefighter Fable for Young Audiences," in a performance that will incorporate the honks and sirens of the Ridge Volunteer Fire Department trucks.

"It's a cool thing, it's quite a spectacle," Silberschlag said.

To allow for those who just want to be outside on a nice night, the seating is broken up into three zones: one for those who come for "serious listening," one for the "casual listening" crowd and another for people interested in "serious socializing." The only rule, said Silberschlag, is if you "happen to wander into the serious socializing area, you have to socialize."

The marketing of the series, funded by many local companies, including defense contractors such as Egan McAllister Associates Inc. and Northrop Grumman, has also tried to keep the image light. One television commercial for the series shows Silberschlag in Amish garb whittling batons in preparation for the concerts; another has him rising from the river.

"You'll never see [conductor] Riccardo Muti [of La Scala in Milan] emerging from a river, but I did," he said. "We've tried, in a way, to make the whole experience a fun experience."

A survey conducted at one of the concerts last year found that about 15 percent of the crowd came from the Baltimore and Washington areas and that the rest came from Southern Maryland. Silberschlag said his goal is to expand the series to where it rivals established music festivals in Aspen, Colo., and elsewhere.

"Right now, we might be the major classical music or symphonic venue in the whole region," he said. "There's no audience like this in Southern Maryland and, arguably, somebody could say in Maryland."

Other performers at St. Mary's this summer include Catrin Finch of Wales, who held the post of "Official Harpist to HRH The Prince of Wales," from 2000 to 2004, and Vonda Shepard, who became famous singing on "Ally McBeal."

Performances take place on the Townhouse Green, overlooking the St. Mary's River. The grounds open at 5 p.m., and concerts begin two hours later. More than 25 vendors sell food, and the fare includes crab cakes and barbecue. Beyond the Friday night concerts, there are movie nights and performances on other nights.

The River Concert Series at St. Mary's College attracts thousands of people each summer.