Hawk Newsome, the co-founder of Black Lives Matter of Greater New York, made the threats following a heated meeting with Adams at Brooklyn Borough Hall.
"If they think they are going to go back to the old ways of policing, then we are going to take to the streets again. There will be riots. There will be fire and there will be bloodshed," Newsome said after the meeting. "There's no way we're going to let some Gestapos come in here and harm our people. We pray for peace, but Black opportunities prepare for the worst."
Adams reacted to the impromptu meeting he had with about a dozen members of the local BLM movement after they marched in front of his office in Brooklyn. He said he won't stand for inflammatory language or threats.
"That's silly. That's silly. I think New Yorkers should not allow rhetoric like that," Adams said on Thursday. "This city is not going to be a city of riots."
The sit-down was behind closed doors but an attendee live-streamed it on Instagram. Adams said he had no problem with the group recording the meeting because he said he has nothing to hide.
"You're holding me accountable? I'm holding you accountable," Adams told the group during the meeting, according to the video. "You're on the ground. Stop the violence in my community. I'm holding you accountable."
Outside Borough Hall after the meeting, Newsome said, "We have people in City Council who can create problems for him. We have people in the streets who can create problems for this administration."
Among other issues, the local BLM group opposes Adams reinstating a new version of the NYPD's anti-crime plainclothes unit that was disbanded after weeks of George Floyd demonstrations.
"They didn't say any of that in the meeting. They know who I am. I ran on a very clear message: my city will not be unsafe," Adams told FOX 5 NY on Thursday morning. That is what I promised to New Yorkers and that is what I'm going to deliver."
During the campaign, Adams, a former NYPD captain, promised to bring back the anti-crime unit to focus on illegal guns.
"We're going to have effective policing that's not going to be heavy-handed," Adams told reporters. "The plainclothes anti-gun unit is going to zero in on guns and gangs. They are going to wear their body cameras."
John Jay College of Criminal Justice Professor Michael Alcazar said the plainclothes units are essential because the criminals do not know who they are.
"We have to keep the bad guys guessing if there's plainclothes officers out there watching them," said Alcazar, a retired NYPD detective.
The Detectives Endowment Association, the union that represents detectives, blasted the threats from the local BLM group.
"We need to support Mayor-Elect Adams' plan to restore public safety and get guns off our streets. NYPD will do the job," union president Paul DiGiacomo said in a statement to FOX 5 News. "Loud threatening rhetoric will not."