COLONIAL BEACH, VA. -- For seven months, a host of nonprofit organizations here enjoyed a new fund-raising bonanza using an old gimmick: big-time gambling.

Skirting Virginia's stringent no-gambling laws, the groups held "Las Vegas night" fund-raisers at a large restaurant and bar that juts out beyond the shoreline (the official border between Virginia and Maryland). By virtue of its location on pilings over the water, Reno on the Potomac is in Charles County, Md., where such gambling activities are allowed, even though it is entered from Virginia, where such activities aren't.

But the Charles County commissioners, arguing that Las Vegas nights have run rampant, have cracked down on such gambling, focusing much of their concern on the Reno, which with 50 employes is the largest private employer in Colonial Beach. The commissioners are meeting today and are expected to adopt stringent rules limiting Las Vegas nights to three times every three months in any one location.

"We were concerned that private individuals were benefiting in the guise of nonprofits," said Charles County commissioner Murray Levy. He noted that the charities were forced to buy food and liquor from the Reno and were being charged $500 a night to rent a room, tables and gaming equipment.

Their concerns echoed those in other Maryland counties, as charity gambling games have grown into a big business and have become embroiled in controversies over free liquor, food and use of credit cards.

The success of the Las Vegas nights at the Reno had astounded local charity officials. Pat Fitzgerald, training officer of the local rescue squad, said, "It brought in $3,000 in the first night." And the fire company topped that, netting $3,500, as much as it previously raised in sponsoring nine nights of bingo, according to Assistant Chief Robert (Teddy) Trowbridge.

Through the spring and early summer, the fire department, rescue squad, school committee and a Colonial Beach revitalization group took full advantage of the Reno's position in Maryland, a geographical quirk that once allowed slot machines to flourish and now permits Maryland lottery terminals there. The Las Vegas night games of blackjack, poker and roulette, commonly used by Maryland charities, raised more than $100,000 for worthy Colonial Beach causes by attracting crowds of Virginians.

But since July, Charles County commissioners have issued permits only one at a time. And the proposals the commissioners will act on today worry charity groups in this faded former river resort of 2,877 people.

Said Trowbridge, a million-dollar Maryland lottery winner who is a maintenance man at the Reno, "Everyone's disappointed. It's gonna hurt us. We just ordered a new truck two weeks ago that cost $220,000."

The instigator of the charity gambling idea, and in no small measure one of its primary beneficiaries, is the Reno's owner, Banks G. Prevatt, a sandy-haired and gravel-voiced former Air Force pilot who admittedly mixes self-interest with what he sees as the public good.

Both a driving force behind revitalization and a lightning rod for controversy, Prevatt, 45, acquired the Reno and 10 adjoining acres in January 1985 for $700,000 with goals to develop the property.

"It's a tired, sleepy little town on a downhill slide since the late '50s when the slot machines went out," he said. "But I could see people sprucing up houses and a fresh crop of retirees and younger people buying property . . . . "

The first organization to benefit from the Las Vegas weekends that Prevatt began in December was one he founded: Citizens Redevelopment Association for the Boardwalk Inc. CRAB eventually bought gaming tables and equipment for $15,000 and charged the other groups the $500 nightly rental fee, a fee that Prevatt said he has now dropped.

Prevatt said CRAB gambling nights alone have raised $70,000. He said CRAB's cash contributions have paid for teen-agers to clean up beach litter on weekend mornings and for new swings and picnic tables for the town. He said that CRAB also has donated $1,000 toward a holding cell for the police station and about $4,000 to promote Colonial Beach over local radio.

But the largest single amount, $20,000, went to build a miniature golf course located directly in front of the Reno on Prevatt's property. "CRAB owns and operates it and the proceeds go to CRAB," Prevatt said.

"Mr. Prevatt is a businessman who's trying to do good for the town and for his businesses," asserted Gary Seeder, a former town council member and the current treasurer of the Colonial Beach Education Foundation, a group that has used the gambling nights to help raise $21,000 toward construction of a $1.8 million high school.

But Mayor Wayne diRosario, 34, accused Prevatt of "riding the coattails" of the nonprofit groups while doing less than he could to upgrade his own establishment. "Mr. Prevatt can be an asset and a liability all at the same time," he said.

The mayor said that the Las Vegas nights "attracted a better quality of clientele" to the Reno, whose reputation for rowdiness predates Prevatt.

He added: "I certainly appreciate that money from his Las Vegas charity nights are going to the rescue squad and the school; that's very impressive. Now, if he could clean up some of the rowdyism, that would be great."

Prevatt acknowledged that there have been problems and said he is considering hiring uniformed guards.

The jurisdictional oddity that has allowed charity gambling inside the Reno has also bedeviled the town police, who must rely on Charles County to enforce the law inside the establishment. Charles County Sheriff James F. Gartland said the Reno is too far away for his officers to patrol regularly.

Despite the rowdiness issue, Tom Hefflin, CRAB president and owner of the Bay Yacht Center marina, insisted "the money made from CRAB for Colonial Beach way, way exceeds the problems {the Reno} brings."

With the county's new limits on charity gambling, Hefflin said, "As far as the Vegas nights, we're 80 percent dead."