Alexandria Mayor James P. Moran Jr. squared off yesterday in an impromptu debate with Northern Virginia Republican leaders over the finances of his congressional campaign, denying GOP charges that he has violated federal laws governing political contributions.

Moran, a Democrat, showed up uninvited at a Republican news conference outside Alexandria City Hall and disputed allegations that he has broken financial disclosure laws in his campaign. The GOP leaders charged that Moran did not tell federal officials about $2,000 worth of expenses that they contend were campaign-related.

The GOP filed a request with the FEC yesterday asking for a probe of Moran's campaign finances.

Moran is attempting to unseat Republican Rep. Stan Parris in Virginia's 8th Congressional District, and their race is expected to be among the most vigorously contested in the Washington area. Although tempers were cool yesterday, Moran's confrontational approach and both sides' rhetoric indicated the contest will live up to its billing.

Fairfax County Republican Chairman Patrick Mullins said Moran has used a state-chartered political action committee "as a slush fund, both personally and politically. We believe he has used it to pay for expenditures that were intended to benefit his race for Congress . . . . There's a past history of irregularities with the way Moran handles his office."

In the early 1980s, when Moran was serving on the Alexandria City Council, he pleaded no contest to a conflict-of-interest charge and resigned from office as part of a plea bargain. He later was elected mayor and a judge set aside the verdict, saying Alexandria deserved a mayor who was not under a cloud.

Calling the Republican charges "silly," Moran warned that "people who live in glass houses ought not throw stones . . . . If that's the way you want to play, we will too."

Late yesterday, Moran released an April letter from the Federal Election Commission to an official in Parris's campaign, questioning the legality of a $2,000 donation from the District's Fraternal Order of Police. Randy Hinaman, Parris's campaign manager, said the FEC determined the donation was legal.

The sparring over campaign finances dates back several weeks, when newspaper accounts revealed that Moran had accepted a $10,000 contribution from an Alexandria developer in January. At that time, several zoning issues that would affect the developer's property were pending before the City Council, which Moran heads.

Moran later voted against a huge office project the developer wanted to build, but also voted against several measures, which the council approved, that dramatically reduced the size and scale of buildings the developer can build on his land.

When reporters asked him about the $10,000 contribution, Moran returned it. He said yesterday that the contribution, which was made to a state-chartered PAC he controls, was disclosed to state officials and that he had done nothing wrong.

Republicans, however, charged that $2,000 from the fund was used to pay for items that will affect the campaign. For example, Moran used money from the fund to join an abortion-rights group; abortion is expected to be a major issue in the race.