It was the day after the great bomb exploded before Mitsuo Tomosawa had a chance to look around his native Hiroshima, which had become, he recalled yesterday, "a city of death."

Walking downtown, toward what had been city hall, he saw a burned-out streetcar filled with dead passengers. Hundreds of bodies were piled under the city's bridges and Tomosawa can still remember seeing helmets with nothing but skulls underneath. "It looked just like hell," he said. "Hell on this earth."

Tomosawa, now an optometrist in Watsonville, Calif., and three other survivors of the 1945 atomic bombing of Hiroshima gave their personal memories of the disaster yesterday to help promote a campaign for a freeze on nuclear arms.

They testified with vivid descriptions in a crowded Senate hearing room before Sens. Mark O. Hatfield (R-Ore.) and Edward M. Kennedy (D-Mass.), coauthors of a joint congressional resolution calling for a weapons freeze by the United States and the Soviet Union.

The resolution, now supported by about 170 members of Congress, has become the focus of a national movement aimed at putting pressure on the Reagan administration.

All four witnesses had been children in Hiroshima when the bomb was exploded over the city on Aug. 6, 1945, by an American bomber crew.

In Tomosawa's memory, the explosion came as a brilliant light comparable to the glow of "a million flashbulbs ignited at the same time." He was knocked unconscious and thrown about 25 feet by the blast, and he awakened to find buildings flattened everywhere.

His first sights were of bodies lying under debris and a hospital room where the bodies were so badly burned that women were indistinguishable from men. A woman in tattered clothing pushed a carriage bearing a baby with a hole in its cheek.

When Kimuko Laskey recovered from the blast, she recalled yesterday, her face was badly cut and her head swollen so severely that she could not open her eyes. Doctors lacked anesthetics and began sewing up her face while she was conscious. Laskey, who now lives in Vancouver, British Columbia, remembered screaming, "Leave me alone," before passing out.

Hatfield, noting that the administration is seeking more funds for a civil defense program, asked the victims if such a program would have done any good in Hiroshima.

"I can't imagine it," said Shigeko Sasamori, now a nurse in Studio City, Calif. "If somebody starts exploding atomic bombs, nobody survives. Nobody."

"It is useless to spend money on it," added Tomosawa.

There are no precise estimates of the number killed in and after the Hiroshima blast. Estimates put the total in excess of 100,000.

The proposal to freeze nuclear arms at current levels drew support at the hearing from three officials of past administrations who criticized the Reagan administration for allegedly seeking a level of nuclear superiority over the Soviet Union.

George W. Ball, a New York investment banker and a former undersecretary of state, praised the Hatfield-Kennedy measure as an effort to change nuclear policy to avoid a catastrophe.

"In government circles," Ball said, "one hears increasing expressions of dangerous nonsense which thoughtful men and women had ruled out years ago--that we should regard nuclear bombs as potential weapons of war and not merely of deterrence."

Rear Adm. Thomas Davies (ret.), once assistant director of the U.S. Arms Control and Disarmament Agency, called the proposed resolution the "only rational approach" and said the United States should cancel plans for deployment of new nuclear weaponry.

Herbert Scoville Jr., former deputy director of the Central Intelligence Agency, said that a freeze on existing arms is verifiable to the extent that there is a "high probability" of discovering any significant violations by the Soviet Union.