In all likelihood there will be no more dramatic moments in Dianne Feinstein's tenure as mayor of San Francisco than the moment when she assumed it.

It was a city still recoiling with horror from the shocj of mass suicides and murders at Guyana that November morning when Feinstein stepped before a microphone at city hall to announce that both Mayor George Moscone and Supervisor Harvey Milk has been shot and Killed.

Suddenly, Dianne Feinstein, a widow of eight months and two-time loser in the race for mayor, had assumed leadership of that peculiar amalgam that makes San Francisco one of the world's most uniquely interesting cities.

"The situation was so stark, so unreal, so terrible that everybody was in a great sense of shock," she remembers. "What really had to be done was, one, reassure people, two, put the bricks back together and three, do what's best because sometimes what happens is that violence begets violence."

Riding in a cab through the streets of Washington yesterday, she was calm and composed and far removed physically and emotionally this springlike day from the chaos of that autumn one.

"I don't really give myself any great credit. I was able to set a note for people which was helpful, but basically the people of San Francisco are just a tremendously resillient and dignified people when the chips are down," she says.

The calm today is in marked contrast to the personal tormoil she suffered after the death by cancer of her husband a year ago and an uncertain political future. She recently annornced plans to be married to San Francisco investment banker Richard Blum, and has become one of the growing pride of female mayors around the country, including those in Phoenix, Wichita, Raleigh, San Jose, Austin and, almost certainly Chicago.

She was in Washington yesterday to testify before two congressional subcommittees. She also renewed her friendship with Ambassador Chai Zemin of the People's Republic of China (San Francisco is one of two cities chosen by the PRC for consulates). She took him to dinner at Sans Souci her first night in town.

A striking woman at 45, she has even features framed by thick black hair only slightly tinged with strands of gray. She wore an impeccably cut three-piece black wool suit, with a white blouse tied at the neck by what San Francisco fashion writers have dubbed "the Dianne bow." The only color in this otherwise black-and-white study was a gigantic ruby engagement ring set amid two equally impressive diamonds.

In a strong and well-moduated voice she is articulate and eloquent as she tells her congressional listeners she wants to talk "briefly from the heart."

A political moderate, she is still "considering" whether she will run a third time for the office she has won by default. It is common knowledge, however, that she recently added the late Mayor George Moscone's campaign manager to her staff.

"The major consideration," she says, "is the city, and the fact that I came in at a time when circumstances warranted, but without a mandate of the electorate."

There are optimistic signs that the constituency has changed since 1975 when, she thinks, had she been a man with the same last name and experience she would have defeated Moscone. Now she says she can tell from the faces, especially women's, that there's pride in how she is doing.

San Francisco's sizable gay community was reassured as well when she acted awiftly to fill the late Harvey Milk's sutervisorial seat with someone else from the gay community.

"It was looked upon as a gay seat," she says. "I felt it was important that violence not interrupt the course of government so I made the decision to appoint someone in the tradition of Harvey Milk."