A nuclear war would have no winners and would cause far greater devastation than most Americans realize, probably destroying all civilization.

This vision, described with scientific fact and with passion, dominated a two-day conference here called by doctors who believe the world has tolorated the danger of nuclear weapons too long.

"We, as physicians, have accepted the arms race as something inevitable," said Dr.Herbert Abrams, chairman of the Harvard Medical School radiology department. "The real enemy is passivity."

"The conditions are not unthinkable. Rather, they are infrequently thought about," said Dr. Howard Hiatt, the dean of Harvard's School of Public Health. "Not to make the effort [to eliminate nuclear arms] would be an insupportable betrayal of ourselves and our children."

Doctors here, desparing of treating the overwhelming casualties of a nuclear war, spoke of this weekend's conference as the beginning of a new effort to arouse American opposition to nuclear weapons.

In scenario after scenario, the doctors and other scientists outlined the immediate and long-term effects of a nuclear war.A 20-megaton attack on Boston would cause total destruction inside a circle with a radius of four miles. Not a single building would be left standing, scientists said.

Within six miles of the blast, only the strongest reinforced concrete buildings would survive.

At least 2.2 million people would be killed by the bomb and subsequent firestorm, Hiatt said. Medical services would be crippled and it is likely that most of those not killed quickly would eventually become casualties.

Survivors would be in a race to erect a life support system before available food and other supplies ran out, MIT physicist Henry Kendall said.

"It is likely that that race would be lost," he added.

Kendall predicted that the world would be likely to return to the level of the Middle Ages, with scattered bands of people competing for scarce resources.

Each one-megaton bomb can devastate a 36-square-mile area, Kendall said. He added that in a scenario for an attack on the United States, using only a small fraction of the Soviet nuclear arsenal, New York City would be likely to get hit not by one bomb, but by 65.

The 650 doctors, nurses, medical students and others who attended the weekend symposium heard over and over that to plan civil defense measures or draw up blueprints for post-nuclear attack medical services is madness.

Using evidence from the World War II conventional firestorm that destroyed Dresden, speakers argued that bomb shelters would become ovens and pressure cookers.

All thought of winning a nuclear war is pernicious, Kendall said. Even calculations of how to survive a nuclear war are dangerous in that they increase the likelihood of wars, warned Dr. Bernard Lown, professor of cardiology at the Harvard School of Public Health.

At the conclusion of the symposium the group of anti-nuclear doctors that organized the discussions, Physicians for Social Responsibility, sent a telegram to President Carter and Soviet leader Leonid Brezhnev expressing alarm at "an international political climate that increasingly presents nuclear war as a rational possibility."

They asked all physicians to join in their appeal for a reduction of U. S.-Soviet tension, a ban on the use of nuclear weapons, and gradual nuclear disarmament.

The world has recognized that biological and chemical weapons are unacceptable, and thermonuclear weapons should be put in the same category, the doctors said. Talking of the future and of Hiroshima, which Yale psychiatrist Robert Jay Lirton called a tiny forestate of the effects of nuclear war, participants described horrors ranging from a smaller heads and higher incidence of mental retardation among people who were in the womb when exposed to radiation at Hiroshima to a postwar world in which the winners would be insects.

Cockroaches and other insects can sustain far larger doses of radiation than man or birds. Without birds, presumably, insects would multiply and swarm over the world's remaining vegetation.

Lifton said people are psychically numbed to the nuclear threat, but that the bomb has worked a terrible deformation on people's minds.

"I think we all lead a double life in the nuclear age," he concluded.