The phrase the president tweeted, “When the looting starts, the shooting starts,” was used by a Miami police chief widely condemned by civil rights groups.

WASHINGTON — An incendiary phrase used by President Trump in a tweet about the protests over George Floyd’s death in Minneapolis appears to have originated in a 1967 news conference held by a Miami police chief long accused of using racist tactics in his force’s patrols of black neighborhoods.

Mr. Trump, in a tweet after midnight on Friday, called the protesters in Minneapolis “thugs” and said: “When the looting starts, the shooting starts.” The phrase was used prominently by Walter E. Headley, Miami’s former police chief, in 1967 as he pledged a no-holds-barred response to a Christmas-season outbreak of violent crime in black neighborhoods that had left three people dead in attempted robberies.

Mr. Headley suggested that his department’s tough tactics had kept Miami calm that year, even as race riots were convulsing dozens of other cities and leaving scores dead.

“We haven’t had any serious problems with civil uprising and looting because I’ve let the word filter down that when the looting starts, the shooting starts,” he said. “We don’t mind being accused of police brutality. They haven’t seen anything yet.”

Hours after Mr. Trump posted his tweet, which was widely criticized for inciting violence, he tweeted about the matter again, saying, in part: “looting leads to shooting, and that’s why a man was shot and killed in Minneapolis on Wednesday night.” He added: “I don’t want this to happen, and that’s what the expression put out last night meant.”

Asked by a reporter at an afternoon news conference whether he was aware of the racist history of the phrase, Mr. Trump said that he’d heard it for years, but he said he was not aware that it had been used by Mr. Headley.

Mr. Headley bolstered patrols of black neighborhoods with a so-called task force of six three-officer patrol cars and five more cars carrying police dogs. The shotgun-equipped officers were authorized under the city’s stop-and-frisk law to detain and search anyone deemed suspicious without a warrant.

“This is war,” he said. “I mean it, every bit of it.”

The chief said his crackdown was aimed at the 10 percent of black people in Miami whom he called “young hoodlums, 15 to 21, who have taken advantage of the civil rights campaign” to commit crimes with seeming impunity.

But national and local civil rights leaders immediately denounced his remarks. Floyd McKissick, the chair of the Congress of Racial Equality, charged that Mr. Headley was “setting up the first fascist state of Miami.”

The president of the Miami chapter of the N.A.A.C.P., Dr. George Simpson, expressed fear that the chief was directing his force “to revert to the law-enforcement practices of 15 or 20 years ago when, in too many instances, to be black was to be guilty.” The field secretary of the group’s Florida chapter, Marvin Davies, said, “This man has no place in a position of public trust.”TRUMP’S TWEETTwitter appended a note to the president’s tweet, saying it was “glorifying violence.”

Mr. Headley repeated the phrase the following August in response to protests in the predominantly black Liberty City neighborhood during the 1968 Republican National Convention, which was being held in Miami. Criticized for not returning from a vacation to address the situation, he said of his officers, “They know what to do. When the looting starts, the shooting starts.”

The chief told Miami legislators in 1967 that his blunt remarks had been partly misinterpreted, and while running the department, he tried to cast himself as a hard-liner on crime without regard to race. Mr. Headley frequently took credit for hiring the first black police officers in the South, in 1944, and for integrating the Miami force. Under his watch the city also eliminated an all-black squad of officers — excepting its white commander — that had been established to patrol African-American neighborhoods.

Mr. Headley’s statements came at the peak of a yearslong wave of racial violence in American cities. The largest and most costly outbreak took place in August 1965 in Los Angeles’ Watts neighborhood after a scuffle between white police officers and two black men who were pulled over in a traffic stop. The ensuing protests led to 34 deaths, 1,032 injuries and $40 million in property damage.

But the disturbances reached a climax in 1967 and 1968, two years that saw more than 150 race riots. In the worst of them, in Cleveland and Detroit, scores of people died and thousands were injured. Protests flared again in the spring of 1968 after the Rev. Martin Luther King Jr. was gunned down in Memphis.

Many white authorities blamed the outbreaks on so-called outside agitators or radical black political groups bent on fomenting violence. But a commission appointed by President Lyndon B. Johnson in July 1967 to study the causes of the protests flatly rejected that the following March, saying white racism was the root of the problem.

The thick report by the National Advisory Commission on Civil Disorders — the Kerner Commission, named after its chair, Gov. Otto Kerner of Illinois — warned that “our nation is moving toward two societies, one black, one white — separate and unequal.”

Miami’s Liberty City erupted in violence again in May 1980, after an all-white jury in Tampa acquitted four white police officers in the fatal beating of a black insurance executive.

In a report on the protests, the Ford Foundation called them unprecedented and likened them to slave uprisings during the Civil War. “Compared to Miami,” the report stated, “the 1960s riots were merely a warning about the hostility that lay beneath the surface rather than the outpouring itself.”