Climaxing a two-day symposium at Harvard on the medical consequences of a nuclear attack on Boston, American physicians yesterday urged President Carter and Soviet Chairman Leonid I. Brezhnev to reject nuclear warfare as a rational possibility and to ban the use of nuclear weapons.

The doctors circulated a petition, which they plan to send to the two leaders and to print in The New York Times, starting that a nuclear war would preclude any civil defense, medical treatment or long-term recovery. "The economic, ecologic and socil fabric on which human life depends would be destroyed," the petition states.

Salvador E. Luria, Massachusetts Institute of Technology (MIT) Nobel laureate in medicine, and three major Harvard physicians signed the petition and spoke at the symposium, organized by July to raise public and professional awareness of nuclear warfare.

Crutches All Around

A 20-megaton nuclear attack on Boston would form a crater a half mile wide and 300 feet deep, Dr.Howard H. Hiatt, dean of the School of Public Health, told an estimated 700 doctors and students Saturday. More than two millions would be killed instantly, and at least five million would die from injuries and radiation. With 80 per cent of the area's medical facilities devastated, and only one doctor for every 17,000 in need of care, there would be no chance of effective medical treatment, Hiatt said.

Speakers said nuclear warfare could lead to epidemics, leukemia in children, psychic numbing, loss of identity, global atmospheric changes, lethal radiation exposure to neutral nations, critical oil and grain shortages and genetic defects for untold future generations.

Moreover, the likelihood of nuclear warfare has greatly risen because of the United State's militaristic attitude and President Carter's 70-per cent increase in nuclear weapon expenditures, George B. Kistiakowsky, Lawrence Professor of Chemistry Emeritus, said yesterday. "Medical treatment would be futile; the only recourse is prevention," Hiatt said.

Specific proposals included an international ban on nuclear warfare, as has been agreed on chemical and biological warfare; ratification of the Strategic Arms Limitations Talks (SALT II) and negotiations for a SALT III treaty; and a popular movement against nuclear weapons, similar to the Vietnam protests.

If physicians don't collaborate with the government and persuade them to abandon nuclear weapons, "our technological capacity might exceed our moral capabilities," Robert J. Lifton, Yale professor of psychiatry, said Saturday