The Defense Department has taken to the pages of the prestigious New England Journal of Medicine to defend a controversial emergency plan to use civilian hospitals for military casualties in a large-scale overseas war, arguing that it is based solely on a "conventional warfare scenario" and not a nuclear attack.

In rebuttal, a physicians' organization attacked the plan on grounds that it ignores the likelihood that such a war probably would "escalate rapidly into a nuclear conflict."

Physicians for Social Responsibility, a group dedicated to warning the public and doctors of the lethal dangers of nuclear war, also fears the American people may be "misled" into thinking that medical measures can deal adequately with a "large-scale" nuclear conflict and "diverted" from what it considers the real issue: the dangerous "potential for nuclear war."

The doctors and the Pentagon "debate" the plan in articles in the current issue of the Journal.

The debate is the result of the Pentagon's top health officials' unusual and unsolicited article aimed at undecided doctors and hospital administrators who read the Journal. The publication offered rebuttal space to the physicians group but neither side was given the opportunity to read the other statement in advance.

The doctors contend that the world's "vast stockpiles of nuclear warheads" and military doctrines make it most likely that a conventional war will "escalate rapidly into a nuclear conflict."

They warn of a potential "holocaust" of "unprecedented devastation" that no emergency medical plan could cope with, and they deride the government's effort as "profoundly unrealistic" and the "height of folly."

Defense Department officials, in turn, charge that successful opposition to the plan could lead to "unnecessary death and suffering for United States servicemen and women" in a war.

At issue is a Department of Defense plan, proposed in 1980, that asks civilian hospitals to set aside 50,000 beds in event of a major war. They would supplement military facilities in caring for wounded military personnel.

The "Civilian-Military Contingency Hospital System" plan has been endorsed by several major medical organizations, including the American Hospital Association and the American Medical Association. DOD says it already has commitments from 433 hospitals to provide 36,000 beds, more than two-thirds the total needed.

But several prominent hospitals, such as those at the University of California and Stanford University, have decided not to participate.

Dr. John F. Beary III and other Pentagon health officials sought to dispel fears of nuclear war, saying there is "basic agreement" that this is "an abhorrent prospect."

But they disputed the assumption that nuclear war is inevitable: "Although nuclear weapons have existed for nearly four decades, they have not been used in any of the conventional conflicts since 1945."

In contrast to the 300,000 military hospital beds available at the end of World War II, the Defense officials said there are now only 18,000. Turning to the civilian sector is preferable to spending $5 billion in constructing new facilities, they said.

The Pentagon disputed the doctors' assertion that the plan was circulated "quietly, without adequate public or congressional debate."

The activist Massachusetts-based medical group has about 10,000 members.

The physicians group contends that it would be unlikely for a nuclear war to remain "limited." It fears an all-out exchange between the United States and the Soviet Union that would "unleash the energy equivalents of 1 million Hiroshima-type bombs."

The resulting "holocaust," it says, could kill 150 million Americans and a similar number of Russians immediately, destroy "virtually every major city in each country," leave lethal fallout which would contaminate vast areas of the world "for generations" and ultimately threaten the survival of life.