“Part of my job is to promote healing,” Mr. Buttigieg told reporters before dropping off the campaign trail in Miami on Friday morning, explaining he needed to engage “members of the community, especially the black community, who are concerned with whether they can trust the police.”

Once he arrived at the protest on a charter flight, around 6:30 p.m., healing was far from the minds of most in the crowd, a mix of bereaved relatives of the dead man, Eric Logan, family members of others injured or killed in police encounters and masked protesters in black waving signs reading, “Who do you call when police murder?”

“You’re running for president and you want black people to vote for you? That’s not going to happen,” a woman shouted.

“Ma’am, I’m not asking for your vote,” Mr. Buttigieg said.

The mayor responded point by point to the 10 demands, agreeing to some — such as requesting the Department of Justice appoint an outside prosecutor — and coolly explaining reasons for rejecting others. “The first demand concerns the firing of police officers,” he said. “The laws of the state are, that’s decided by a board of safety.”

When he was interrupted, he replied, “Could I continue responding?”

Mr. Logan’s mother, Shirley Newbill, was ushered in front of the mayor.