The trial of a libel suit in which controversial presidential candidate Lyndon H. LaRouche Jr. is seeking $150 million from NBC Television opened yesterday in U.S. District Court in Alexandria amid tight security.

Attorneys for both sides spent much time in pretrial motions raising issues about confidential sources, courtroom security and the possibility that witnesses and jurors might find themselves harassed or threatened.

LaRouche, who will be listed as an independent candidate on the presidential ballot in 19 states including Virginia, was not present, and his attorney, Odin P. Anderson, said his absence was primarily out of concern for his security.

LaRouche failed earlier this year to secure Secret Service protection in light of his national candidacy and has maintained he is a target for an assassin, a claim U.S. District Judge James C. Cacheris addressed by placing two U.S. marshals in the courtroom instead of the usual one and allowing two LaRouche aides, referred to by the judge as "security people," to remain in the back of the courtroom.

LaRouche's presence in the courtroom "would be an obvious setup," Anderson told a reporter. Anderson said LaRouche, who is expected to testify at the trial, was campaigning yesterday, but he said he didn't know where.

The suit stems from segments of two NBC programs broadcast earlier this year. The three-time presidential candidate was described on the air as the anti-Semitic leader of "a political cult" that uses "a kind of political nastiness that hasn't been seen in America in some time."

The segments, aired on the NBC Nightly News and the now-defunct First Camera news magazine show, were replayed in court yesterday. They allege that LaRouche and his followers engage in smear campaigns, have intimidated reporters who inquire about them, use lawsuits as weapons against the press and, this year, sabotaged campaign appearances by Democratic presidential candidate Walter F. Mondale.

Included in the "First Camera" segment is testimony from persons identified as LaRouche defectors that LaRouche followers are often "armed and dangerous."

Attorneys for NBC, which has filed a countersuit, asked Judge Cacheris to instruct jurors to report immediately any threatening letters or phone calls, which NBC alleged in its broadcasts are LaRouche's signature. NBC attorney Thomas Kavaler told Cacheris a key witness had been harassed by anonymous calls on the eve of the trial.

LaRouche's suit alleges that the program segments were false and malicious and accuses NBC producer Pat Lynch and reporters Brian Ross and Mark Nykanen of conspiring to defame LaRouche in an attempt to destroy his political aspirations and influence on government officials, including President Reagan. LaRouche will have to prove the network was malicious and had a "reckless disregard for the truth."

The Anti-Defamation League of B'nai B'rith is also a defendant in the case, the result of a charge by an ADL leader in one of the broadcasts that LaRouche is "a small-time Hitler."

Dr. John Grauerholz and Muriel Mirak -- LaRouche followers -- took the stand yesterday to deny that LaRouche's organization encourages anti-Semitic views or a cult following. When asked under cross-examination by Kavaler whether they believe that NBC favors drugs, both said they believed that NBC, The New York Times, The Washington Post and Walter Mondale, among others, operate as part of a "drug lobby" that promotes the use and sale of drugs.

NBC's countersuit charges that LaRouche aides used "dirty tricks" against the network by impersonating senatorial aides in phone calls to NBC and by impersonating NBC reporters in a call to at least one U.S. senator.

NBC attorneys said yesterday they will call as witnesses people on the various layers of their news-gathering organization and show unedited interviews in an attempt to explain decisions made before the airing of a major news piece. The issue has been highlighted recently by the multimillion dollar libel suit against CBS News by retired Army General William Westmoreland. made before the airing of a major news piece. The issue has been highlighted recently by the multimillion dollar libel suit against CBS News by retired Army General William Westmoreland.