Mayor Bill de Blasio spoke to reporters a day after Trump said he would send law enforcement to New York, Philadelphia, Chicago and other cities in a federal crackdown on anti-racism protests sparked by George Floyd's death in Minneapolis police custody on May 25.
A Republican, Trump made a point of saying the mayors of the cities on his list were liberal Democrats, underpinning concerns the threat was politically motivated. Federal agents last week began cracking down on protests in Portland, Oregon, where the mayor is also a Democrat.
De Blasio said Trump's threat may prove to be bluster, but added the city would challenge any deployment in the courts. He said unidentified officers grabbing citizens from the streets "appeared to violate basic constitutional rights."
"This president blusters and bluffs and says he's going to do things and they never materialize on a regular basis, so we should not overrate his statements, they are so often not true."
In Portland, federal officers fired tear gas at demonstrators in a protest on Monday night that spilled into the early hours of Tuesday. Local TV said the protest was the city's largest in more than 50 nights of demonstrations against police brutality.
Video showed protesters pulling down fencing around the federal courthouse. It also showed hundreds of people dressed in yellow who said they were mothers and fathers demanding that the federal agents withdraw.
White House spokeswoman Kayleigh McEnany said the dispatch of agents would be justified by a federal statute affording the Secretary of Homeland Security the power to deputize agents to protect federal property and people on that property, and suggested there was room for interpretation.
"Under the law, we believe that agents can conduct investigations of crimes committed against federal property or federal officers, and in the case when you have someone shooting off a commercial-grade firework and running across the street, we don't believe that extends past our jurisdiction," she said.
Philadelphia Mayor Jim Kenney predicted the deployment of federal agents would backfire by exacerbating tensions at a time when the cities are still grappling with the spread of COVID-19.
"That the White House seeks to impose federal involvement in this way, after months of abrogating its responsibility to lead a federal response to COVID-19, is both ironic and offensive," Kenney said in a statement.
Kenney said he would "use all available means to resist such a wrong-headed effort." He was no more specific.
Oregon Attorney-General Ellen Rosenblum sued the US Department of Homeland Security last week and asked a federal judge to grant a temporary order blocking its officers from what she called unlawful detentions that lacked probable cause.
"Ordinarily, a person exercising his right to walk through the streets of Portland who is confronted by anonymous men in military-type fatigues and ordered into an unmarked van can reasonably assume that he is being kidnapped and is the victim of a crime," the lawsuit said.
The American Civil Liberties Union has also sued the department, saying its officers used violence against journalists and legal observers.
After Trump cited Detroit as a possible target for a deployment, Michigan Governor Gretchen Whitmer, also a Democrat, assailed him for what she characterized as a "politically motivated threat."
"There is no reason for the president to send federal troops into a city where people are demanding change peacefully and respectfully," Whitmer said in a statement. "Quite frankly, the president doesn't know the first thing about Detroit."