Leesburg used to be a model of old-time Virginia gentility, a place where most faces were familiar and townspeople traded good-natured small-town news over the lunch counters on King Street. Now they look up and down the Safeway aisles before they whisper the latest news, and they are afraid to speak their mind at public meetings.

Leesburg's new uneasiness is centered on Lyndon H. LaRouche Jr., the extremist political leader who moved his organization's headquarters and several hundred supporters from New York to Loudoun County last year.

While LaRouche originally was regarded by the local residents as a curiosity or a temporary irritant, it is dawning on them now that LaRouche and his empire -- including more than $2 million worth of real estate, a newspaper, a radio station, a printing plant, a typesetting firm and large research and editorial offices -- are there for the long haul.

Many county residents are not eager to have LaRouche and his supporters for neighbors. They cite, for example, an incident last fall when LaRouche supporters passed out leaflets at churches one Sunday accusing several prominent civic leaders of being "part of the international drug lobby" and "a treasonous nest of gossiping liars."

"I feel as if I've come into contact with something evil," said Agnes Harrison, a 40-year county resident and past president of the Leesburg Garden Club who was described in the leaflet, signed by LaRouche, as a member of "a highly organized nest of Soviet fellow travelers." Harrison added: "I feel he has brought fear, distrust and suspicion to the county."

LaRouche, 63, is the leader of a close-knit worldwide group known for its apocalyptic pronouncements and shifting ideological stances. LaRouche, who has run three times for president, was a Marxist for decades until the mid-1970s, when he moved his organization to the far right. Now his core group, called the National Caucus of Labor Committees (NCLC), espouses a mishmash of ideas but is mostly praiseworthy of President Reagan and the U.S. military buildup.

The LaRouche group routinely uses threats, verbal attacks and questionable tactics to dissuade former members, critics and the news media from discussing the organization, said LaRouche ex-associates and experts on the group. LaRouche denies the charges.

Last month the LaRouche group sent national Democratic Party leaders into shock after two supporters of the group won the party's nomination in Illinois for secretary of state and lieutenant governor. Democratic gubernatorial nominee Adlai Stevenson III is trying to set up an independent slate after refusing to run with the LaRouche people.

Meanwhile, people in Loudoun wonder what goes on behind the fences of the 170-acre estate west of Leesburg where LaRouche lives. The $1.3 million property is guarded by armed sentries and lit by bright white lights all night. LaRouche is convinced that he is targeted for assassination by Libyan hit squads and others. The property, purchased by an Oklahoma oil executive who was a financial contributor to LaRouche's 1984 presidential campaign, consists of a large manor house with 14 rooms, eight fireplaces and five baths, as well as other houses.

Many locals also are puzzled by the comings and goings of the several hundred supporters and employes of the LaRouche-affiliated groups, who live in houses and apartment buildings throughout the county. Most of their offices operate 24 hours a day. Around 2 a.m. most nights, a shift change at the main editorial offices on South King Street sends crowds of employes home and brings a new crew in. Inside, at the headquarters of Campaigner Publications Inc., rooms are cramped with elaborate computers and communications equipment, said one person who visited.

"They come and go all night long," said the neighbor about a group of LaRouche associates who, like most county residents, wanted to remain anonymous for fear of retribution. The neighbor added that the group can be heard singing and playing classical music when not at work.

County Supervisor Frank Raflo said he fears that the LaRouche supporters' sometimes unpredictable behavior will harm efforts by the fast-growing county to attract tourism and economic development.

"We're constantly selling the quality of life in Loudoun County," Raflo said. "The quality of life has been severely diminished because of Lyndon LaRouche."

The LaRouche people bristle at the criticism.

"There's been no majority opinion in the county against LaRouche," said Edward Spannaus, a top LaRouche aide. He added that the complainers have "allied themselves with the drug lobby." That term, he said, referred mostly to New York journalist Dennis King, who has broken many stories about LaRouche and who once wrote an article for High Times, a promarijuana magazine.

While the LaRouche supporters do not mix easily with most residents, they do have daily dealings with many Leesburg merchants. It has created a dilemma -- most store and restaurant owners interviewed said they share the suspicions of others in Loudoun, but few will turn down the business.

At midday, LaRouche supporters flock to the shops and small restaurants in downtown Leesburg, with its historic red brick buildings and spruced-up window fronts.

"You're getting factions built up" among local business owners, said one longtime Loudoun resident. "There's no open war, but you gradually take sides. Are you willing to come out, or are you going to be quiet and take their money?"

"I was reluctant to take the LaRouche supporters' business at first," said one restaurant owner who estimated they spent more than $5,000 in his place last year. "But if he wants food, I'll give it to him."

LaRouche associates also have become a major factor in the town's tight rental market, renting dozens of apartments and homes and considerable office space.

A year ago, when most residents first learned from news accounts about the LaRouche group's reputation for harassing critics, few county residents would express criticism of the group publicly. But many became less timid last fall when a controversy erupted over a LaRouche group's zoning request to operate a summer camp on a 64-acre tract in a remote part of western Loudoun. Many residents feared some sinister intentions. Hundreds of people packed public hearings in opposition, and dozens spoke publicly.

LaRouche photographers took photographs of people addressing the hearings, and the verbal and written attacks began against Raflo, Harrison and others who spoke against the zoning proposal. LaRouche described the opponents as "a selection of the county's less patriotic residents."

A further chill descended last month when a LaRouche organization filed a $2 million libel and slander suit against a Leesburg businessman, Steve Dabkowski, for statements he made about LaRouche supporters to a local television station. Raflo and Leesburg Mayor Robert E. Sevila have formed a defense fund for Dabkowski -- the first organization set up in Loudoun to oppose the LaRouche organization.

Spannaus said Raflo is "bringing nationwide discredit to the county" and that the LaRouche group's investment in the area shows the organization is committed to Loudoun.

Among its enterprises, the LaRouche organization has established numerous offices, a Leesburg typesetting company and a Sterling-based printing plant.

In addition, it has set up a weekly newspaper, the Loudoun County News, prompting concern from some Loudoun residents over the power the organization is gaining in the county's news media. The group says the News' circulation is 8,500. It is distributed free and by subscription.

The paper has a column on bird-watching and covers local issues such as property assessments and the county's gypsy moth eradication program -- and largely without the shrill tone in the LaRouche group's other publications.

But some residents complain that the News sometimes hypes events. A recent attempt by county officials to place restrictions on a small number of business owners in western Loudoun prompted headlines like "3,000 Businesses to Be Shut Down?" About 100 angry business persons packed the next county supervisors meeting. Raflo said the paper had needlessly "inflamed" people who would not have been affected by the action.

The News' advertisements also have angered some Loudoun business owners. Some merchants said the most recent issue carried advertisements that they did not approve and that appeared to be lifted from other publications.

John F. Sleeter, owner of a produce market, said he "absolutely did not authorize" the ad for his business. "I'm very upset about it," Sleeter said. A spokesman for the Leathercoat restaurant in nearby Fauquier County said it had not authorized its ad.

County Commonwealth's Attorney William Burch said he has received several complaints about unauthorized ads in the News but that there is no law against the practice.

Spannaus denied that any ads were printed without approval and said the complaints were generated by LaRouche's enemies.

Two months ago, associates of the group bought a radio station, WTRI-AM, for $270,000, according to Federal Communications Commission records. From its studio in Brunswick, Md., the country music station's broadcasts reach Loudoun, Frederick and Montgomery counties.

"We're still a daytime country music station," said program director Tom Lesser, who worked at the station long before it was bought by a partnership headed by longtime LaRouche associate Allen Salisbury. Lesser said the LaRouche supporters frequently send tapes and news articles for the station's news show but have not pressured him to use the items.

"Allen looks at it as a business," Lesser said, adding that the new owner plans changes in programming and news. While the 500-watt station is number five in the area's ratings, "Allen's got a bigger idea: He wants us to be number three or two or one."

Spannaus noted that the group's investments have brought 250 new jobs to the area.

"The area appealed to us as a relatively quiet, peaceful . . . atmosphere, as compared to the degradation of New York," Spannaus said. "Absolutely, we're here to stay."