A federal grand jury here is investigating complaints that a former aide to president-elect Ronald Reagan has mailed tens of thousands of invitations to an extravagent, two-day inaugural celebration at the D.C. Armory highlighted by apparently unsubstantiated plans for appearances by both the new president and vice president elect.

The invitations, embossed with an official looking, gold "Presidential Inaugural" seal and mailed to 40,000 people across the country, were issued by the Presidential Inaugural Foundation, organized by James E. Johnson, 53, an evangelical minister who was former Gov. Reagan's secretary of veteran affairs in California and also one-time assistant secretary of the Navy under John W. Warner, now a Republican senator from Virginia.

An attorney representing the official Reagan inaugural committee said yesterday that neither Reagan nor vice President-elect George Bush plans to attend anything other than the events scheduled by the official committee.

"It [the Johnson invitation] looks pretty official, but this is not part of the official inaugural ceremony. We want to make sure that the public is not misled," said Roger Clark, an attorney for the official inaugural committee. He added that the committee is considering seeking a court order to stop Johnson from issuing any additional invitations.

Johnson, in a prepared statement yesterday, described his enterprise -- formally called "Presidential Inaugural Celebration. . . With Love -- as a "nonsectarian, nonpartisan, nonpolitical, joyful, prayerful" tribute to the glory of God and the United States. He said he intended it to be an "alternative to the official inaugural events which sometimes degenerate into drunken brawls."

Johnson's attorney, John F. Snyder, said yesterday he had "extensive discussions" with various federal investigators and is convinced there is no basis for any criminal action against Johnson. Johnson declined to comment on the grand jury investigation.

Johnson appeared before a grand jury in U.S. District Court here yesterday and turned over several cartons of records of both this inaugural foundation and its parent National Heritage Foundation, which Johnson's lawyer described as a charitable, tax-exempt organization set up to raise money for religious and politically conservative causes.

Various sources said yesterday that the grand jury inquiry was prompted by a variety of complaints, including one from the Carter White House, received by U.S. Postal inspectors in October, when the bulk of the invitations were mailed.

The protest from the White House came after a high ranking member of the Carter staff received two unsolicited invitations to Johnson's inaugural event, described as a "historic Celebration featuring nationally and internationally known speakers and singers as well as political figures." No entertainers or political figures were named in the invitation.

The White House concern focused on two lines in the program for the celebration with listed a "Planned visit" by the newly elected president and vice president.

Deputy White House counsel Michael H. Cardozo sent a strongly worded letter to Johnson on Oct. 27 in which he warned Johnson that it was "deceptive . . . to advertise unequivocally" that the president- or the vice president-elect would appear at the celebration since neither Carter nor Mondale, if elected, planned to attend.

In the letter, Cardozo asked Johnson to "refrain from continuing this misrepresentation" and asked that corrective steps be taken to alert those who had already received invitations. Cardozo said copies of the letter were made available to the U.S. Secret Service, postal authorities and the office of U.S. Attorney Charles F. C. Ruff.

Sources indicated that the grand jury investigation, expected to continue next week, is focused on the narrow question of whether Johnson, a U.S. Civil Service commissioner during the Nixon Administration, had any assurances that whoever was elected president or vice president would attend the events, scheduled for 8 p.m. on Jan. 19 and 20, which is Inauguration Day.

Snyder said his client "assured himself beforehand with verbal commitments" from high-ranking people in the presidential campaigns that the winners would attend his celebration.

Invitations asked for a donation of $135 per person to attend events on both nights. It was learned yesterday that Johnson may have collected more than $300,000 as a result of the invitations, mailed to a box at the main post office here, most of which was reportedly spend on mailing costs and other expenses.

In his statement, Johnson said he had made "no provision for payment for myself" but allowed for "reasonable" expenses in connection with the event. f

Sources said yesterday that Johnson placed a $1,000 deposit with officials at the D.C. Armory last month to rent the huge facility and told armory officials he expected as many as 16,000 people to attend the events.

Snyder said he was confident the investigation would clear Johnson, whom he described as an "evangelical, born-again Christian" who "makes Jimmy Carter look like a pagan."