On the first day of July, more than 20 people sat in a circle under an unrelenting sun outside City Hall in Utica, N.Y. City employees peeked from behind shutters and through office windows. A 19-year-old man, Thuong Oo, stood in the middle of the group, shouting into a megaphone.

The police had killed Mr. Oo’s 13-year-old brother, Nyah Mway, two days earlier and he was angry. Late on June 28, an officer shot Nyah after the boy had been seen with what turned out to be a pellet gun and was tackled to the ground.

“I went to the hospital and they said they had a shootout with my brother,” Mr. Oo told the crowd. “But I watched the video and it said different things, so it doesn’t make any sense.”

Many of those encircling Mr. Oo were members of the Karen, a refugee ethnic group from Myanmar, and almost all them were young. “Take my hand not my life!” one demonstrator’s sign said. “I still want to go to school.”

Older Karen residents stayed on the periphery. They were supportive — some carried their own signs — but they were quieter than their younger counterparts.

Refugee families make up a quarter of Utica’s 60,000 residents. About 8,000 are Karen, and the demonstrations prompted by Nyah’s killing have revealed a dynamic that is common in immigrant communities: Young people who grew up in the United States, speak English well and understand the country’s systems often become the public activists.