Thousands of mentally ill people are being forced off the Social Security disability rolls without adequate evidence or examination, witnesses told the Senate Committee on Aging yesterday.

Many are under 24-hour supervision, confined to hospitals, hallucinatory or "virtually comatose," the committee was told.

"They're doing delicate surgery with a chain saw," said Maryland Attorney General Stephen H. Sachs in criticizing Social Security Administration procedures for reviewing tens of thousands of mentally ill people to see if they are still too sick to work. Social Security repeatedly has denied improper actions.

Sachs and others said the SSA had insisted until a recent midwestern court ruling that if a person can get along fairly well in a home situation he should be presumed able to hold a job without an on-the-job or similar evaluation.

Dr. Arthur Meyerson of the American Psychiatric Association said the SSA requires that in disability findings certain symptoms must be evident at the time of the examination, although many severely crippling mental symptoms occur intermittently, and that patients should be evaluated on the basis of a case history, not one interview.

Committee Chairman John Heinz (R-Pa.) said testimony by the General Accounting Office, Sachs, doctors and officials from Pennsylvania and other states presented evidence of a "callously unfair system" that has produced a "virtual holocaust" against the mentally disturbed and ill.

He said Social Security, as part of a congressionally mandated review of people on the disability rolls for physical and mental impairments, had looked into 663,000 cases between January, 1981, and August, 1982, and issued initial orders of termination of benefits for 335,000, including 86,000 mentally disabled.

Although not all terminations were improper, Heinz said, there was evidence that "Individuals involuntarily committed to a mental institution have been told they are able to work, even while they remain committed against their will."

The GAO's Peter J. McGough said the agency had studied 159 recent terminations, 40 in great detail with the aid of the GAO's full-time clinical psychologist and mental health adviser.

"She concluded that in 27 of the 40 cases, the individuals could not function in their daily living without support and could not work in a competitive or stressful environment," he said. "In an additional 13 cases, she concluded that more medical or psychosocial information or trial work experiences were needed to make an informed decision."

One 34-year-old man with a 61 IQ, overt signs of psychotic behavior, a hysterical personality orientation and other disabilities was terminated because he was able to watch television, read, walk his dog and do some cooking, McGough said.