President Trump’s admonition that the police should not be “too nice” while transporting suspects drew laughter and cheers from a crowd of officers on Friday, but police officials swiftly made it clear they did not find the words funny.
From New York to Los Angeles, law enforcement authorities criticized the president’s remarks. Experts worried that his words could encourage the inappropriate use of force. A defense lawyer even signaled that he might use video of the speech in court.
The criticism online started shortly after Mr. Trump’s comments, which came at an event in Brentwood, N.Y., which was intended to support the police in their fight against La Mara Salvatrucha, or MS-13, a gang that has been accused of several murders on Long Island.
After calling for more immigration officers to help arrest the gang members, Mr. Trump told officers, “Please don’t be too nice.”
“Like when you guys put somebody in the car, and you’re protecting their head, you know, the way you put your hand over” their head, he said, putting his hand above his head for emphasis. “I said, ‘You can take the hand away, O.K.?’”
The president’s remark was denounced by police officials and organizations, including the International Association of Chiefs of Police, the Police Foundation and Steve Soboroff, one of the civilian commissioners who oversees the Los Angeles Police Department.
“What the president recommended would be out of policy in the Los Angeles Police Department,” Mr. Soboroff told The Los Angeles Times. “It’s not what policing is about today.”
Michael Harrison, chief of the New Orleans Police Department, said in a statement on Saturday that Mr. Trump’s comments “stand in stark contrast to our department’s commitment to constitutional policing and community engagement.” The department is one of more than a dozen since 2009 to agree to make reforms under the direction of a federal monitor.
The Suffolk County Police Department, in New York, which had officers at the speech, responded within two hours.
“As a department, we do not and will not tolerate roughing up of prisoners,” it said on Twitter. The department “has strict rules & procedures relating to the handling of prisoners,” it said in another post. “Violations of those rules are treated extremely seriously.”
The White House did not return messages seeking comment on Saturday. Some supporters rallied to Mr. Trump’s defense, including the police group Blue Lives Matter, which said on Twitter that the remark was obviously a joke.
Mr. Trump’s words were particularly sensitive in Suffolk County.
The county’s Police Department agreed to federal oversight in 2013 after allegations of discrimination against Latinos. And a former chief, James Burke, was sentenced in November to 46 months in federal prison for beating up a man who had stolen a duffel bag containing pornography and sex toys out of his car and then for attempting to cover up the assault and other misdeeds. Other officers also pleaded guilty in that case.
On Saturday, Matthew Tuohy, a criminal defense attorney in Suffolk County, said that while he believed the president’s remarks were intended to be humorous, the reaction from the officers in the crowd “exemplifies the mind-set and today’s culture” in law enforcement on Long Island.
Mr. Tuohy said that hypothetically, he would “absolutely utilize” video of the speech to bolster a civil case of a suspect who wanted to sue the department for brutality, or to try to discredit the testimony of an officer in a criminal case.
In New York City, the president’s comment and the reaction from the crowd recalled the days of former Mayor Rudolph W. Giuliani, who was accused at times of ignoring inappropriate behavior by the police. Before becoming mayor, he opposed a proposal to create a civilian board to review police conduct and famously rallied a rowdy demonstration of officers against the idea.
Maya Wiley, the chairwoman of the city’s Civilian Complaint Review Board, condemned Mr. Trump’s speech, saying in a statement that it was “shameful, dangerous and damages the progress our city has made toward improving police-community relations.”
In a statement on Saturday, the New York police commissioner, James P. O’Neill, said the department’s training and policies about the use of force “only allow for measures that are reasonable and necessary under any circumstances, including the arrest and transportation of prisoners.”
“To suggest that police officers apply any standard in the use of force other than what is reasonable and necessary is irresponsible, unprofessional and sends the wrong message to law enforcement as well as the public,” he added.
The department declined to say whether it would remind officers about not mistreating suspects. A spokeswoman for the Nassau County Police Department said the department had no plans to send a reminder, while the Suffolk County Police Department did not return messages seeking comment.