The crowd cheered as Larry Pratt, gun lobbyist and conservative Virginia state delegate defeated last November, bounded down the aisle and accepted the chairmanship of the Fairfax County Republican Party's Springfield district meeting Tuesday night.

An attempted coup was under way.

A faction of the Fairfax Republican Party -- labeled the extreme New Right by its opponents -- this week launched a well-organized campaign to capture the party apparatus. The attack, led by Pratt, former Del. John Buckley and other well-known conservatives, took many party officials by surprise and gave the conservatives control of the party in several sections of Fairfax.

The conservatives have set their sights on the party's county chairmanship, which will be decided at a convention April 17. Their victories, however, have left some party regulars speculating anxiously about the conservatives' hit list, with potential Republican victims ranging from Supervisor Marie Travesky to 8th District Rep. Stanford E. Parris.

"It was a power play, and in their own eyes I'm sure they're strutting around this morning feeling very proud," said Joan Jacobs, current county party vice chairman and the moderate's candidate for the chairmanship. "It's sort of an indication of what we might expect in the convention."

The conservatives deny any designs on Parris, who if nominated at the district convention in May faces a tough rematch against Democrat Herbert E. Harris II this fall. The conservatives say they only want to inject new life into the Republican Party, which fared poorly in Fairfax and Virginia last year. They accuse the "liberals" in the party of spreading rumors to help preserve their own power.

The conservatives' candidate for party chairman, Ben Partin, is a former Air Force brigadier general, unsuccessful 1980 candidate for state delegate and founding board member of a private Christian school. "You have a few of these militant socialists in the high schools who spend their time on anti-Christian, anti-religious teaching," said Partin, explaining his role with the Engleside Christian School. Partin describes himself as a "forward-looking, compassionate conservative."

"I happen to think there's room for all thoughts, that we don't have to cleanse the party," said Jacobs. "If the community sees the party as very, very, conservative and the community is not all very, very conservative, you have a clash."

To a Democrat, neither Fairfax faction would look particularly liberal; both strongly support President Reagan.

But which faction triumphs at the county convention could influence the type of candidates the Republicans field during the next two years. Moderate Republicans predict that the insurgents would alienate voters, while the insurgents -- who concentrate on social issues such as abortion and prayer in public schools -- promise to return the party to a purer conservatism.

The Republicans staged their mass meetings in each of the county's eight districts this week, picking delegates for not only county, but also congressional conventions scheduled for May. Pratt was elected temporary chairman as the Springfield meeting began at West Springfield High School on Tuesday night, giving the insurgents control of the rules and pace of the session for the rest of the night.

Scores of Republicans from the insurgent faction filed as delegates just before the deadline 12 days ago, giving moderates little time to respond once they caught on. Many Jacobs supporters, like Republican Supervisor Nancy Falck, said the insurgents raised abortion and other emotional issues to turn out supporters who generally give little time to the party.

"I began hearing people saying, 'Someone told me to file, what's a county committee?' " Falck said. "I figure the people who do the work ought to have a chance to have a say in the party."

Partin rejected Falck's criticism. "You're always going to have an in-crowd of people who are going to regard the organization as their private domain," he said. "I think that's destructive."

The battle has exacerbated a long-running feud between Supervisor Travesky, a Jacobs supporter, and her Republican colleague on the board, Chairman John F. Herrity, whose staff worked for Partin. It also caused bad blood between Falck and Herrity, who had maintained a cordial if not effusive relationship.

Falck accused Herrity of meddling in her Dranesville district, and she "lit into him" behind the board room two weeks ago, said one supervisor. Falck said yesterday that she was skeptical of Herrity's claims of neutrality since his staff had been so active. "I said, 'If it wasn't a sin of commission, it was a sin of omission, and why didn't you tell them to cut it out?' " Falck said. Herrity declined comment yesterday.

Meanwhile, rumors continue to circulate about the insurgents' plans: Pratt is preparing to run against Republican Del. Warren Barry, it is said, or against Travesky, or against the current chairman of the party in the 8th Republican District, Robert Cunningham; former Del. Robert Thoburn is preparing to run against Parris, as he did in 1980, or against Del. Robert Harris. And so it goes.

Neither Pratt nor Thoburn could be reached yesterday, but most party officials said they do not think anyone would challenge Parris or his 10th District Republican colleague, Rep. Frank Wolf. Instead, one knowledgeable Republican said, the conservatives will use the party machinery to challenge moderate Republicans and Democrats in the state legislative races this year and to strengthen themselves for 1983 and 1984.

"They lost Buckley and they lost Pratt and they took kind of a bath in the last election," he said. "So I think they've done what anybody does when they've taken some losses -- they're going to fundamentals."