Hundreds of FBI agents and former agents are expected to gather at the U.S. District Courthouse here this morning in a "silent vigil" of support for three former FBI officials indicted on charges of approving illegal break-ins.

Former acting director L. Patrick Gray III, and two high-ranking aides, W. Mark Felt and Edward S. Miller, are scheduled to be arraigned at 9:30 on charges they conspired to violate the civil rights of friends and relatives of members of the radical Weather Underground in the early 1973s. It is alleged they approved break-ins by agents from the FBI's New York office.

One source familiar with the planned demostration predicted yesterday that 300 agents from FBI headquarters, 250 from the Washington field office and 200 from New York are expected.

Other agents are expected from Baltimore, Alexandria, Norfolk, Richmond, Newark, Philadelphia, Detroit and New Haven.

One organizer, who asked not to be identified, predicted the demonstration would be similar to one held in New York last year in support of John J. Kearney, an FBI supervisor who was the first person indicted in the ease. Prosecution of him was dropped as part of the Justice Department's resolution of the case.

Edward P. Morgan, a Washington attorney and former agent, aid he will make a brief statement of support in behalf of former FBI agents. "We're hoping for a very quiet, dignified affair," he said.

New FBI Director William H. Webster said during a Senate appropriations subcommittee hearing yesterday that the planned vigil is not sanctined by the FBI and that all participants will be acting on their own time and expense.

Webster said he felt the demonstration was "a fraternal show of support" for "those caught in a change of standards" and not a disagreement with new federal guidelines that prohibit such warrantless break-ins.

But today's scheduled congregation of FBI agents and former agents, coupled with the recent criticisms by J. Wallace LaPrade, an assistant FBI director being disciplined for allegedly lying during the investigation, and complaints by more than 60 New York field agents also facing discipline, show that the controversy over Attorney General Griffin B. Bell's resolution of the FBI inquiry is far from over.

Bell, told a Senate appropriations subcommittee yesterday that he has had "a lot of second thoughts" about his decision not to indict LaPrade on charges he lied to a federal grand jury about his role in the break-ins.

LePrade, former head of the FBI's New York office, has leveled a series of charges against Bell and the Carter administration since being told last week that he is being fired instead of prosecuted.

He challenged Bell to a television debate and accused the Carter adminstration of politicizing the bureau. He also said this administration is continuing warrantless investigations similar to the break-ins conducted in search of the Weather Underground.