With a minute of silent prayer, the release of 1,500 doves and the offering of ritual flowers to 138,690 dead, Hiroshima marked the 40th anniversary this morning of its devastation in history's first nuclear attack.

About 55,000 Japanese and foreigners gathered for ceremonies in Peace Memorial Park, a 30-acre oasis of trees, monuments and relics of the bomb, code-named "Little Boy," which exploded 1,900 feet overhead at 8:15 a.m., Aug. 6, 1945.

An elderly women wiped away tears as the assemblage fell silent at 8:15 a.m. to mark the precise time the bomb fell. Several hundred people threw themselves to the ground in a "Die-In" at the "A-Bomb Dome," the sole ruin that the city has preserved. Other people prayed in apartments, by riverbanks and in nursing homes.

Hiroshima Mayor Takeshi Araki appealed for nuclear disarmament. "Today's hesitation leads to tomorrow's destruction," he said. "The fates of all of us are bound together here on earth. There can be no survival for any without peaceful coexistence for all."

More than 50 smaller memorials and demonstrations that will go on all day began to unfold in other points around the city to climax its year-round, international drive to keep memories of the bomb alive and bring about nuclear disarmament.

They include a convocation known as the First World Conference of Mayors for Peace Through Inter-City Solidarity, which has brought officials from about 95 cities in Japan and abroad to Hiroshima.

In recent days, the city has been filling up with peace activists, high school students, a few international celebrities and much of the leadership of Japan, including Prime Minister Yasuhiro Nakasone.

Many people attending the mayors' conference had one-on-one sessions with atom bomb survivors, who told their stories in graphic and sometimes tear-provoking detail. Some of the mayors clasped hands with the bomb victims.

"Absolutely shattering," was how actor Jack Lemmon, one of the foreign visitors, described his experiences so far. Lemmon said he had never been an activist but plans to speak out on nuclear disarmament when he returns to the United States.

The form of today's ceremonies is the same as in past years. But size and impact will be larger. More than 50 foreign news organizations are covering it. ABC anchor Peter Jennings began his nightly broadcast with the dome in the background.

The main ceremony began with the dedication of a list of names of the bomb's victims. The names of about 4,200 persons who survived it in 1945 but have died in the past year were formally added to the list, to make 138,690. Thousands more are believed to have died in the blast but never have been identified.

Then, dignitaries including Nakasone and the speakers of the upper and lower houses of the Japanese parliament strode to the A-Bomb Cenotaph, the main peace monument in the peace park, to lay wreaths of flowers.

At 8:15 a.m., the gathering fell silent for a minute of prayer and to contemplate the ringing of the "Peace Bell." It was followed by the release of the doves, which flew off in a great swarm into cloudy skies, and a declaration of peace by Mayor Araki.

A rendition of the Hiroshima Peace Song by a chorus of 600 and brass band of 200 members followed.

Peace activists also conjured up a variety of theater to press the theme of "No More Hiroshimas."

A trolley car dating from the day of the attack, one of four said still to be in service here, then carried 40 bomb survivors through the streets.

"When I lie down on the ground," said one of the "Die-In" organizers, Hitomi Kumanaka, "I feel I can communicate with the dead spirits of the bomb victims. That gives me strength to take action."

Elswhere, a special memorial for victims who were newspaper employes was to be unveiled. Ceremonies were to be held for the students of particular schools and members of the postal union.

This afternoon, rival antibomb groups loyal to the Japanese Socialist and Communist parties will sit down together for a two-hour session in a city gymnasium, one of the few occasions when they cooperate.

In the evening, Leonard Bernstein will conduct a special Hiroshima Requiem.

Afterward, thousands of tiny lanterns with candles alight will be set afloat from bridges in the city to make a final gesture of commemoration for the day.

Preparations have been under way for weeks. After today, many participants will move to Nagasaki, scene of the history's second and final nuclear attack, staged on Aug. 9, 1945.