A federal grand jury in Boston has indicted political extremist Lyndon H. LaRouche Jr. on a charge of conspiracy to obstruct justice in the latest round of indictments against his Leesburg-based organization.

The indictment -- issued Tuesday but unsealed yesterday -- is the first criminal charge ever against LaRouche. It comes after 19 years of periodic federal investigation of him, starting in the late 1960s, when he was a Marxist, and extending into the middle 1970s, when he was allied with neo-Nazis.

Authorities charged him for what prosecutors have said was his group's "flagrant" defiance of law in trying to scuttle a federal probe of alleged financial frauds by fundraisers for his 1984 presidential campaign.

The indictment said LaRouche conspired with top aides to send three associates under investigation to Europe, to harass the prosecutor leading the probe and to refuse to cooperate with the grand jury, which last year indicted 13 LaRouche associates on charges of credit card fraud and obstruction of justice.

"The investigation was necessary," said Frank L. McNamara Jr., the U.S. attorney in Boston, "in order to maintain the public's confidence in the ability of the grand jury to perform its vital function even when faced with a concerted effort to obstruct and impede that function."

If convicted, LaRouche, 64, faces up to five years in jail and a $250,000 fine.

LaRouche, who is currently in West Germany, has agreed to return to the United States for a preliminary hearing Tuesday in exchange for prosecutors' recommendations that he not be held pending trial.

He spent seven months in Europe until a Monday appearance before the Boston grand jury that indicted him.

He returned to West Germany hours after his testimony.

LaRouche has denied the charges.

Yesterday he said in a statement that the Justice Department is a tool of Soviet intelligence, and that the probe is part of "the dirtiest and biggest scandal to hit Washington in the entire postwar period."

The indictment caps a 32-month investigation that started in October 1984, and included a series of unexpected twists, including the alleged burning of subpoenaed documents by LaRouche associates, LaRouche's failed attempt to stop the probe by publicizing details of his associates' past dealings with some Reagan administration officials, $21 million of fines against LaRouche groups for contempt of court, the bankruptcy of three LaRouche corporations and the subsequent shutting of about 10 LaRouche offices around the country.

The latest surprising development came this week, when LaRouche returned from West Germany to testify before the federal grand jury.

His attorneys had strongly advised against his appearance on the grounds that it might help prosecutors indict him.

Another federal grand jury in Alexandria has been investigating LaRouche and his associates for about a year for alleged income tax improprieties and fraudulent loans.

In addition, grand juries in Loudoun County and in New York state have issued indictments of LaRouche associates in connection with alleged fraudulent fund-raising. But before today, the man whom law enforcement authorities say they sought all along had never been charged.

News of LaRouche's indictment caused jubilation in Leesburg, where LaRouche has engendered hostility since he and at least 250 associates moved there three years ago. "People are slapping each other on the back and just going crazy out here," said one investigator there.

"We are very encouraged by this action and hope to be just as successful here in Virginia," Loudoun County Sheriff John Isom said in a statement. He added -- in a tongue-in-cheek allusion to LaRouche's frequent references to America's revolutionary period -- that "like the American revolution . . . that which was started in Boston, we intend to finish in Virginia."

LaRouche's name was added to those of his associates previously indicted on the conspiracy charge, which includes 47 separate criminal acts in which they collectively took part, the indictment said.

The charges grow out of a probe of allegations that LaRouche group fund-raisers made unauthorized withdrawals from credit card accounts of LaRouche contributors.

According to the indictment, LaRouche helped plan the group's scurrilous personal attacks on William Weld, who previously was U.S. attorney in Boston and is now a top Justice Department official.

A LaRouche associate's notebook seized in raids on his Leesburg offices last October showed that LaRouche told an aide that "we are going to stall {the investigation}, tie them up in the courts . . . just keep stalling," the government charged.

The probe was delayed until spring 1986, when two LaRouche associates' victory in the Democratic primaries in Illinois -- and subsequent negative publicity about a Rouche -- brought pressure on Justice officials to get it moving.

The final stroke came Monday. Sources said LaRouche, who is running for president, volunteered to travel from Germany to Boston to address the grand jury.

He turned over voluminous documents, including a new autobiography. Several sources who know him say LaRouche is given to arrogance and believed he could persuade the grand jury to drop the probe.

The sources said his behavior resembled his actions before federal jurors in Alexandria in a 1984 libel trial. LaRouche sternly lectured them about his ideas and his place in world history, but the jurors -- who found against him -- later said he repulsed them.

"He thinks he can talk his way out of anything," said one law enforcement source.