President Donald Trump is to send "a surge" of federal security forces to US cities in a crackdown on crime.
Chicago and two other Democratic-run cities are being targeted in the Republican president's move, amid a spike in violence.
But federal deployments in Portland, Oregon, amid ongoing unrest have been controversial. The city's mayor was among those tear-gassed at a protest.
Law and order has become a key plank of Mr Trump's re-election bid in November.
Since the death on 25 May of an unarmed black man, George Floyd, in police custody in Minneapolis, Minnesota, there have been protests - sometimes descending into civil disorder - in scores of US cities.
In Portland, which has seen more than 50 days of demonstrations, Mayor Ted Wheeler was tear-gassed by federal agents while attending the city's protest on Wednesday night.
Speaking to a New York Times reporter, the Democrat said the tear-gassing was "an egregious overreaction" by federal officers, and that he had seen "nothing that provoked this response".
"This is not a de-escalation strategy, this is flat-out urban warfare," Mr Wheeler said. Other local officials have similarly criticised the president's tactics, saying he has escalated tensions in the city.
Meanwhile, gun violence has spiked in metropolitan areas including New York City, Philadelphia, Los Angeles, Chicago and Milwaukee.
What is Operation Legend?
The operation announced by President Trump is named after a four-year-old boy, LeGend Taliferro, who was shot dead while sleeping in his family home in Kansas City in June. The boy's mother joined the president at Wednesday's announcement.
The operation will see agents from the FBI, Marshals Service and other federal agencies work with local law enforcement, according to the US Department of Justice.
Mr Trump - whose opinion poll numbers have been slumping amid a coronavirus-crippled US economy - said: "This rampage of violence shocks the conscience of our nation."
The president, who accuses Democrats of being weak on crime, said: "In recent weeks there has been a radical movement to defend, dismantle and dissolve our police departments."
He blamed this for "a shocking explosion of shootings, killings, murders and heinous crimes of violence". He added: "This bloodshed will end."
US Attorney General William Barr, who was with Mr Trump, said they had sent about 200 federal agents to Kansas City, Missouri. They would send a "comparable" number to Chicago and about 35 others to Albuquerque, New Mexico.
Mr Barr said the officers would be involved in "classic crime fighting", unlike the deployment of Department of Homeland Security agents which were sent to "defend against riots and mob violence" in Portland.
However policing in the US is the responsibility of states, and governors and local officials have resisted the deployment of federal agents.
Oregon Governor Kate Brown has called it "a blatant abuse of power," and Mayor Ted Wheeler "an attack on our democracy."
Chicago Mayor Lori Lightfoot said on Tuesday: "We welcome actual partnership, but we do not welcome dictatorship."
What is going on in those cities?
In Portland protests have been held every night since Floyd's death in May. In recent days demonstrators have clashed with federal officers sent earlier this month to the city to guard a court.
The agents have been accused of driving in unmarked vehicles around Oregon's biggest city while wearing military fatigues and arbitrarily arresting a handful of demonstrators.
In the latest incident in Chicago, at least 14 people were shot outside a funeral home in a suspected gang-related attack.
The city has seen 34% more homicides than last year, according to the Chicago Sun-Times.
A 2018 FBI survey found the violent crime rate in Albuquerque was 3.7 times the national average. The rates of murder and rape were more than double the national average that year.
Kansas City, meanwhile, is on track to record its most homicides ever in 2020, according to the Kansas City Star.
Defending the intervention of federal troops on Monday, President Trump blamed the violence in those Democrat-controlled cities to the fact that they are "all run, really, by the radical left".
Crime-fighting or electioneering?
Analysis by Tara McKelvey, Kansas City
Many people in Kansas City say the president's plan is not what this place needs. Kansas City leans Democratic, and many are opposed to the president's policies and object to his aggressive law-and-order mandate.
Traditionally crime-fighting programmes are popular during election campaigns, and critics of the president say this initiative is directed more at his conservative supporters in the rural heartland than those who live in Kansas City and other urban areas.
Mr Trump's critics say the initiative is not really an effort to fight urban crime, claiming it is simply a way to drum up enthusiasm among the president's base of supporters before the election.
Why is crime spiking?
Criminal justice experts note the crime wave has struck in the middle of a coronavirus epidemic that has ushered in mass unemployment.
The Police Executive Research Forum, a law-enforcement nonprofit, told ABC News that one factor behind the uptick was various jurisdictions granting early release to inmates to prevent the spread of Covid-19 in prisons.
New York City police have blamed the spike there on a bail reform law that requires judges to release defendants awaiting trial on serious crimes.
Stay-at-home orders have made it difficult for officers to conduct community outreach, while Floyd's killing has fuelled distrust of law enforcement in the areas they serve, say criminal justice experts.
NYPD Chief Terence Monahan said hostility towards his officers over the last couple of months had emboldened some people who believed "that cops can't do anything anymore".
There is also a suggestion that some law enforcement could be taking a step back in frustration over anti-police rhetoric.
According to CBS News, members of Chicago's police union recently encouraged officers in a text message not to volunteer for overtime, to limit arrests and to call in sick to make a point to city hall about the need for law enforcement.