Several leading Virginia Democrats are trying to block the expected appointment of Justin W. Williams as permanent U.S. attorney for Alexandria.

The Democrats say they believe Williams, who has been serving as interim U.S. attorney, is more of a Republican than a Democrat.

Justice Department officials confirmed yesterday that Williams has emerged as the leading contender for the job. He is said to have strong support from both outgoing U.S. Attorney General Griffin B. Bell and his successor, Benjamin R. Civiletti, who takes over from Bell today.

Williams' emergence has angered state Democratic leaders, who say his nomination could do harm both to the party -- which is trying to recover from a decade of electoral defeats -- and to President Carter's chances of carrying Virginia in next year's presidential election.

Seven leading Democrats -- including Northern Virginia Reps. Joseph Fisher and Herbert E. Harris, state party chairman Richard L. Davis and former Lt. Gov. Henry Howell -- sent a joint telegram earlier this week to Carter, Civiletti and national party chairman John White objecting to Williams, though not by name.

"we strongly urge you to appoint one of the qualified Virginia Democrats submitted by our party . . . not a Republican or unacceptable person," the telegram read.

Many of the Democrats used stronger language yesterday to describe their displeasure.

"This is just a slap in the face of the Democrats of Virginia," said Sandra Duckworth of Fairfax County, a member of the Democratic National Committee."There's no excuse for it and I'm furious."

"I don't know if you could call me upset, or disturbed or dissatisfied," said Davis. "I suppose any of those verbs would do."

As for Williams, his secretary said he would have no comment on the issue. He also declined to identify his party affiliation, if any. Virginia does not require voter registration by party.

What particularly angered some Democrats was that until a week ago they said they were being told by White House political operatives such as Tim Kraft that the nomination would go to a qualified Democrat. Two prominently mentioned names were Fairfax attorney John T. Schell and Richmond attorney Karen Kincannon, a former assistant state attorney general.

"There seems to be a good deal of confusion between the White House and the Justice Department over this," said Harris.

Kraft did not return repeated phone calls yesterday. But a source with close ties to Kraft said Virginia politicians may have misunderstood that while White House aids favored a Democrat, Attorney General Bell was likely to have the last work on the nomination.

"Nine times out of 10 the president tends to follow Judge Bell's advice," said the source. "It can be particularly frustrating to the people in the White House who are trying to look after the president's political interests."

The Carter administration faces a similar contest with Democrats in filling the District of Columbia's U.S. Attorney's post. The Justice Department reportedly is ready to nominate former special Watergate prosecutor Charles F. C. Ruff instead of Togo D. West Jr., who is the unanimous choice of city Democratic leaders.

In both cases, Bell and the Justice Department have passed over candidates with strong political qualifications to tap others they feel have stronger backgrounds as prosecutors. The uproars illustrate the difficulty the Carter administration has had in fulfilling his campaign pledge to take politics out of the U.S. Attorney's office.

The post has long been considered a political patronage plum, especially in Virginia where its $46,600 annual salary is $1,600 more than the state's attorney general earns. It became available in June when William B. Cummings, a Republican appointed in 1975 by President Ford, resigned.

The previous leading contender for the job, Richmond attorney Matthew N. Ott Jr., surprised the Justice Department by withdrawing from consideration earlier this month. Ott, a Democrat with no strong party credentials, was opposed by party officials such as Davis and Lt. Gov. Charles Robb.

But Ott was preferable to Williams as far as party leaders are concerned. They're angry in part because, unlike in D.C., the submitted a list of more than a half-dozen qualified Democrats for the post rather than attempt to pressure the administration into accepting one name. All appear to have been rejected.

Without demonstrating influence when it comes to federal patronage posts, state Democrats say they'll be hard-pressed to attract new recruits to the party.