More than 500 FBI agents, clerks and friends gathered somberly outside U.S. District Court here yesterday in a muted show of support for former FBI acting director L. Patrick Gray III and two other high ranking FBI officials as they pleaded innocent to criminal conspiracy charges inside.

The agents, some of whom had come by chartered bus from New York and other eastern cities for the unusual public display, parted ranks on the courthouse plaza and applauded as Gray, former FBI acting associate director W. Mark Felt and former assistant director Edward S. Miller marched one by one into the court building to be formally arraigned.

"All I can say is God bless everyone here," said Felt from the top of the courthouse steps.

The gathering of agents at the courthouse yesterday was within blocks of the Capitol, Justice Department and White House where in the turbulent years of the Vietnam war many of these same agents had watched and infiltrated the ragged ranks of radical demonstrators.

But those at yesterday's event stood quitely in crisp three-piece suits. There was no singing, no chanting, no flag burning and those present even refused to call the event a "demonstration."

"This is not a demonstration. This is a show of support," said Nick F. Stames, agent in charge of the large and influential Washington field office.

Agents organizing the carefully orchastrated gathering had consulted with General Services Administration officials beforehand and avoided the use of placards, banners or other appearances of demonstration. No permit was required for the gathering, according to GSA officials.

Three former FBI officials were indicated last week on charges of conspiring to violate civil rights by authorizing illegal break-ins of homes by FBI agents searching for radical Weather Underground fugitives in the early 1970s.

"Let this event today clearly reflect our personal commitment and assure the American people that our flight against those (Weather Underground) terrorists was nothing more than our just and sworn duty," said special agent Patrick Connor of the New York field office in one of the two brief speeches to the crowd.

Stames of the Washington field office told a reporter at the gathering that new congressional guidelines are needed to direct FBI agents involved in sensitive security investigations so that they will no risk criminal charges similar to those now facing Gray, Felt and Miller.

"What was acceptable back in the '60s is not acceptable now," he said."What is acceptable now might not be acceptable in the '80s. We need guidelines."

The presence of the FBI agents and their support for the three indicted men obviously affected the defendants and their families.

Felt's eyes filled with tears as the well-wishers shook his hand and patted him on the back as he moved through the crowd towards the courthouse for the arraignment. The wives and other members of the three defendants' families also appeared near tears.

"All I can say is God bless everyone here," said Felt from the top of the courthouse steps.

As the crowd milled about quietly outside, Gray, Felt and Miller stood in a fourth-floor courtroom and formally pleaded innocent to the charges before Judge Charles R. Richey.

The judge instructed the defense attorneys and Justice Department prosecutor Barnet D. Skolnik to work out a schedule for pretrial motions and set a status call on the case for May 12 before Chief Judge William B. Bryant.

U.S. Marshal Jerome J. Bullock then accompanied the three defendants to the basement cellblock for fingerprinting and photographing, a routine procedure for criminal defendants.

Thirty minutes after entering the court building, the three left, again to the applause of the crowd outside.

Gray, looking pale and grim, pushed his way through the waiting reporters, muttering "no comment" and stumbling briefly on the steps. Miller and Felt waved to well-wishers. Then, like Gray, left in waiting cars.

While the defendants were inside the courthouse, the atmosphere of a subdued class reunion settled over the crowd on the plaza facing Constitution Avenue NW as agents met long-departed friends and old acquaintanceships were renewed.

"Hey, Bobby. How you doing? . . . John, remember me? We worked together on that bank robbery in Wheeling."

Talk of jobs, transfers and new assignments were interspersed with discussions of the baseball season and tips on how to enclose the back porch.

"It's a shame we had to meet under these circumstances," said one agent to an ex-colleague. "But I'm seeing faces today I haven't seen in years."

While reporters at the scene generally agreed the crowd numbered about 500, Deputy Chief Robert Klotz of the D.C. Police Department estimated the number of 700, and FBI organizers of the event put the number at 900 to 1,000.

Many agents in the crowd would not talk with reporters and most of those who did talk refused to give their names.

"I don't have a name," said an agent from New York in a brown suit who appeared to be a principal organizer of the event.

"We're playing this very low key for the three stand-up guys (Gray, Felt and Miller)," he said.

Agents willing to talk said they had taken vacation time to come to the gathering, and those from New York and other cities had paid their own way.

Most agents and ex-agents interviewed said they felt surreptitious entries by FBI agents without a court-ordered warrant were justified against the Weather Underground organization because of the crisis atmosphere of the time the Weather Underground's publicly announced policy of blowing up buildings and committing other viilent acts.

"I think (Gray, Miller and Felt) were justified in (authorizing) what they did, if in fact they did anything," said one retired agent from New York.

Said Stames of the Washington field office: "I could be prosecuted for things I'm doing now" if legal restraints on warrantless investigations are tightened.

He said his office is currently conducting a number of sensitive investigations that have the approval of the President and the attorney general without a court warrant. He did not elaborate.

Another agent who said he had retired after more than 33 years in the bureau told a reporter, "Ten years ago, we didn't have people indicted." At that time, he said, what was done "was necessary for the good of the country. Now it's wrong."

Agents "resent prosecutorial action against people for doing what was necessary . . .," he said.

"They were doing their job," said another retired agent. "We were trying to locate the underground that was disrupting this country."

There have been "attacks against the bureau" since Watergate, said still another agent. "This has been a sore point at the bureau for years.

"How can people do their work when they're attacked?" he asked.

"I spent thousands of hours in this courthouse," said an employe of the Washington field office as he waited on the steps for Gray, Felt and Miller to come out. "I never thought I'd see this, though."

One former high official of the FBI who arrived at the courthouse and received applause was John J. Kearney, a retired New York field office supervisor. Attorney General Griffin Bell recently dropped criminal charges against Kearney because he said he decided to prosecute only those officials at the highest level in the bureau.

Most of the persons attending yesterday's gathering were young to middle-aged men, but there were also a few women. One woman with her son in a backpack said she was there because her father and brother are both employed by the FBI. Another woman said she came to show her support because her husband works for the bureau.

Deputy Chief Walter Evanoff of the Federal Protective Service, the police arm of GSA, said organizers of the event "talked with us extensively" about arrangements.

"Because there were to be no signs, no placards and that kind of thing, we did not consider it a demonstration, so no permit was required," he said.

Sec. 1507, Title 18, of the U.S. Code of Laws makes it a criminal offense to picket or parade near a federal courthouse with the intent of influencing or interfering with the administration of justice. Organizers said yesterday, however, their gathering did not amount to a picket or parade and their intent was not to influence the Gray-Felt-Miller case but just to show support for the defendants.

"This is not a demonstration or a rally or a vigil," said Evanoff. "It's not anything. It's just a group of individuals standing silently. If anything, it's like a funeral."

Contributing to this story were Washington Post staff writers Laura A. Kiernan and James Lardner.