Political extremist Lyndon H. LaRouche Jr. turned himself in to federal authorities in Boston yesterday for arraignment on obstruction of justice charges. He pleaded not guilty.

A federal magistrate ruled that LaRouche be held in lieu of $20,000 bond. Within minutes of the hearing, an 82-year-old LaRouche supporter from Pennsylvania paid the sum by check, court officials said.

U.S. Magistrate Robert Collings also decided that LaRouche should surrender his passport on Sept. 8. Until then, Collings said, LaRouche must restrict his travels to the United States and West Germany, where his wife maintains citizenship, unless he submits a written itinerary the week before any trip elsewhere.

LaRouche returned to the United States from West Germany on Monday, four days after his federal indictment was unsealed.

LaRouche's indictment followed similar charges against 13 associates on charges of credit card fraud -- in connection with his 1984 presidential campaign -- and obstruction of justice.

Prosecutors say he and his associates conspired to send witnesses to Europe, hide subpoenaed documents and stall a federal probe in other ways.

LaRouche supporter Edmund Haigler, an engineer from Hatboro, Pa., secured the $20,000 check that released LaRouche. If LaRouche doesn't appear for the Sept. 21 trial, Haigler would forfeit the money.

Under federal rules, a magistrate had to find that someone putting up bond money must be close enough to the defendant that the defendant would feel morally required to appear for trial.