“At one point, one group may emerge because they’ve got greater population and another group feels pushed out — feels like they don’t have any voice anymore,” said the Rev. A. Cecil Williams of Glide Memorial Church. “It involves a kind of power shift. That, of course, creates some of the tension.”

The rapidly deteriorating climate has alarmed local leaders. The president of the Board of Supervisors, David Chiu, noted that on Wednesday, hundreds of Chinese lined up at a board meeting to tell stories of assaults and intimidation, sometimes without clear motivation, by young African-Americans.

Two days later, a young black man, Amanze Emenike, 21, said he was 12 when he heard older boys talking about why they singled out Asian and Latino immigrants: they would not report the crime and had no gangs to back them up. On Friday morning, on a Hunters Point hilltop with a breathtaking view of the Bay, Mr. Emenike and his sister, Sherry Blunt, 22, recounted their “spree” of crime against Asian and Latino immigrants several years ago.

By the time he was 15, Mr. Emenike said, he and his brother, Armani Bolmer, would get up at 5 a.m. to rob Mexican day laborers who got off the 23 Monterey bus from the Mission district.

They began to single out Chinese, he said, because they had more money. In 2006, they stalked a Chinese man at the last Muni stop, robbed him, and were arrested hours later.

Black civic leaders say they are troubled by the rash of crimes and have repeatedly emerged at rallies alongside Chinese to show solidarity. They also point out that black-on-black crime remains, statistically, a far bigger problem.

Image Amanze Emenike, 21, and his sister, Sherry Blunt, 22, talked openly about why they had singled out Asians for robberies. Credit... Jim Wilson/The New York Times

But at these Chinese rallies and vigils, beneath the megaphone-amplified din of positive rhetoric, there are worrying murmurs about revenge, said Henry Der, who was the executive director of Chinese for Affirmative Action, an influential Chinatown organization, for more than two decades.