Political extremist Lyndon H. LaRouche Jr., returning from a seven-month stay overseas, is scheduled to appear voluntarily next week before a federal grand jury in Boston that indicted 13 of his associates in a continuing investigation of alleged obstruction of justice by LaRouche associates, according to sources familiar with the probe.

Law enforcement officials have said for months that they believe LaRouche -- whose group has been hit with three rounds of federal and state indictments as well as an involuntary bankruptcy filing -- would not return because of his organization's mounting legal problems.

However, associates of LaRouche, who has declared his fourth candidacy for the presidency, maintained he would return. LaRouche was expected to reenter the country by yesterday, and is expected to hold a campaign news conference today in Rochester, N.H., where he grew up.

"He's anxious to campaign," said LaRouche group spokesman Dana Scanlon. LaRouche is running as a Democrat, although almost all party officials around the country strongly denounce him. Scanlon said LaRouche wants to distinguish himself from the other Democratic presidential candidates, whom he has called "the seven dwarfs."

Scanlon said she had "no information" about LaRouche being scheduled to appear before the Boston grand jury. LaRouche's Boston lawyer could not be reached for comment. Assistant U.S. Attorney John Markham, handling the case in Boston, declined to comment.

It could not be learned precisely what LaRouche would be questioned about in his scheduled appearance before the Boston grand jury. But one of the main federal charges facing his associates and five LaRouche-affiliated organizations is conspiracy to obstruct justice. Prosecutors often have said LaRouche is in charge of everything that happens in the group, including its activities in legal cases.

Last October, the Boston grand jury indicted the LaRouche associates for credit card fraud in connection with raising funds for LaRouche's 1984 presidential campaign. The obstruction charges involve alleged efforts by LaRouche associates to burn or hide subpoenaed documents, and send witnesses out of the country to make them unavailable to investigators.

Three of the 13 persons indicted in Boston have not been found, and prosecutors said they are in Europe.

The LaRouche group has said the federal charges are a result of a conspiracy by the Soviets, narcotics dealers and anti-LaRouche factions of the White House.

LaRouche also is being investigated by a federal grand jury in Alexandria looking into income tax improprieties. Law enforcement officials have said they have a criminal case against LaRouche involving personal income taxes, but LaRouche has denied the allegations.

Officials widely believed that LaRouche, who left the United States in December, would stay out of the country indefinitely and fight extradition if indicted. LaRouche's associates are known to have looked into extradition procedures in various nations.

A source close to LaRouche said that one reason for LaRouche's return is that "he's still serious about his political goals." The source said LaRouche associates have concluded that for LaRouche to gain credibility in this country, he must be willing to face legal obligations here.

In recent months, LaRouche has traveled in Europe and Latin America, but has spent most of his time in Wiesbaden, West Germany, where his group has its European headquarters.

With LaRouche's scheduled return to his home town today, he will be encountering a place that he had found alienating as a youth. Rochester is a small, failing factory town, and in his autobiography, LaRouche, whose father managed a shoe manufacturing company, described his early years as "bitterly boring and gray." He said he was a "semi-outcast" who read philosophy books.