Kathy Boudin, one of the best-known radicals of the 1960s, went on trial today on robbery and murder charges in connection with the $1.6 million holdup of a Brink's armored truck in 1981.

Three persons, including Boudin's husband, David Gilbert, were convicted last year of robbery and second-degree murder in connection with the incident in which two policemen and a Brink's guard were killed.

The defendants, claiming to be political prisoners, were sentenced to 75 years to life in prison after refusing legal counsel.

They called the robbery an "expropriation" to finance a Republic of New Africa, a separate nation in the southern United States.

Boudin had been arrested shortly after the robbery at a police roadblock where a U-Haul truck carrying part of the cash was stopped.

Her lawyers say she was a passenger in the truck, but was unarmed.

"Kathy has stoutly maintained her innocence," said Boudin's attorney, Leonard Weinglass. "The only question is whether we can get a fair trial."

The slender, intense woman of 40 who sat quietly in a Westchester courthouse today has been an almost legendary figure in the annals of the American left.

Daughter of the prominent civil rights attorney Leonard Boudin, she became an antiwar and welfare rights activist and a member of the Weather Underground, an offshoot of the Students for a Democratic Society (SDS).

In 1970, three youths were killed in an explosion in a secret Greenwich Village bomb factory.

Police said Boudin was seen running from the scene. She went into hiding.

Despite a nationwide search by the FBI, Boudin remained underground until her 1981 arrest. By then, many of her associates in the Weather Underground had surfaced, either to face charges in connection with demonstrations of the time, or to take up normal lives, marrying, having children, going to law school, teaching and writing.

Two fugitives charged in connection with the Brink's robbery are still at large.

Today, Orange County Judge David S. Ritter, who is trying the case in Westchester because of prejudicial publicity in Orange and Rockland counties, refused to sever Boudin's trial from that of another defendant in the case, Samuel Brown.

Brown, 43, became an FBI informer in a federal conspiracy trial connected to the case.

Weinglass, Boudin's attorney, said Ritter's denial of severance could result in a mistrial and would deprive Boudin of the testimony of Brown, who, Weinglass said, would state that she was not at the robbery.

Judge Ritter also denied Boudin's request for a hearing on the issue of whether blacks are underrepresented in the Westchester County jury pool.

Boudin's attorneys argued that extraordinary security measures surrounding the trial would prejudice the jury.

The high-rise courthouse is encircled with concrete barriers, and three metal detectors were set up in the lobby to screen those entering.

"No one can come to this building without thinking they are coming to a combat zone," Weinglass said. "This isn't Beirut. This is White Plains. Kathy has been a model prisoner and a model person in court. There is no justification for this."

Boudin's attorneys also protested the fact that she is being held in Rockland County, across the Hudson River, requiring her to travel more than an hour a day in manacles.

Boudin's father, who is financing and actively participating in her defense, said her sudden removal last week from the Orange County jail amounted to "a classic case of kidnaping" by Rockland County deputies.

The removal stems partly from the efforts of Rockland County, where the robbery occurred, to save money.

Rockland already had to pay Orange County, where the first trial was held, $3.5 million in security costs.

Now Westchester estimates the Boudin-Brown trial will cost more than $6 million during its expected six-month duration.

Rockland County District Attorney Kenneth Gribetz has said he will seek consecutive terms of 25 years to life for Boudin on three counts of felony murder and one count of robbery. The trial may turn in part on evidence that, Gribetz alleges, links Boudin to the crime scene.

It includes microscopic particles of glass found on her clothing that may have come from the shattered Brink's truck windshield and one piece of her hair electrostatically fixed to a duffle bag containing part of the money in the back of the truck.

More than 11,000 prospective jurors, three times the usual number, have been called in the case. Today, only 27 of 200 brought before the judge were asked to return for questioning.

The rest were disqualified because of personal circumstances or expressed prejudice in the case.

Boudin has been held without bail for 2 1/2 years. During much of that time, she was unable to have physical contact with her son, Chesa, who was 14 months old at the time of her arrest. Her lawyers and parents called the conditions inhumane, and Boudin is now allowed to visit with her son, although he may not bring toys to the visits.