BOSTON, NOV. 2 -- The conspiracy trial of a former security consultant for political extremist Lyndon H. LaRouche Jr. got under way today with the prosecution and defense attorneys calling the defendant a liar.

Opening arguments were heard in U.S. District Court in the trial of Roy Frankhouser, a former member of the Ku Klux Klan and American Nazi Party.

Officials also announced Frankhouser has agreed to testify as a prosecution witness against LaRouche and 11 codefendants in return for immunity from further prosecution in the case -- a move that could hurt LaRouche's defense because of Frankhouser's inside knowledge of the group.

LaRouche and his associates face a later trial on obstruction of justice charges in connection with the same case.

Frankhouser, 47, of Reading, Pa., is charged with conspiracy to obstruct justice by interfering with an FBI investigation into alleged credit card fraud by LaRouche associates.

Assistant U.S. Attorney John Markham said LaRouche supporters followed Frankhouser's advice when they reportedly burned records, hid witnesses and refused to cooperate with the grand jury probe.

"This is a man who is a liar, a con man and a cheat," said Markham. "He is a liar . . . because he told the LaRouche group -- in order to get a job -- that he had important contacts in the government."

Owen Walker, Frankhouser's lawyer, told the jury his client was indeed dishonest with the LaRouche group.

"To some extent, at least with respect to the lying and being a con man, the charges are not incorrect," Walker said. "Mr. Frankhouser did engage in a bit of fakery."

But Walker said Frankhouser did not try to thwart the grand jury.