The Trump administration, stepping up its crackdown on an alleged crime wave in the America's capital, has named the head of the Drug Enforcement Administration (DEA) to be Washington DC's “emergency police commissioner" with all the powers of the police chief.

The move is significant in that it increases national control over the city as part of the federal government's law-enforcement takeover.

Attorney General Pam Bondi said in a directive issued on Thursday evening that DEA boss Terry Cole will assume “powers and duties vested in the District of Columbia Chief of Police.”

The Metropolitan Police Department (MPD) “must receive approval from Commissioner Cole” before issuing any orders, Bondi said.

It was not immediately clear where the move left Pamela Smith, the city's current police chief, who works for the mayor.

DC Attorney General Brian Schwalb responded late on Thursday that Bondi’s directive was “unlawful”, arguing that it could not be followed by the city’s police force.

“Therefore, members of MPD must continue to follow your orders and not the orders of any official not appointed by the Mayor,” Schwalb wrote in a memo to Smith, setting up a potential legal clash between the heavily Democratic district and the Republican administration.

Washington Mayor Muriel Bowser wrote on social media that “there is no statute that conveys the district’s personnel authority to a federal official”.

Bondi’s directive came hours after Smith directed MPD officers to share information with immigration agencies regarding people not in custody – such as someone involved in a traffic stop or checkpoint.

The Justice Department said Bondi disagreed with the police chief’s directive because it allowed for continued enforcement of “sanctuary policies".

Bondi said she was rescinding that order as well as other MPD policies limiting inquiries into immigration status and preventing arrests based solely on federal immigration warrants. All new directives must now receive approval from Cole, the attorney general said.

The police takeover is the latest move by US President Donald Trump to test the limits of his legal authority to carry out his agenda, relying on obscure statutes and a supposed state of emergency to bolster his tough-on-crime message and his plans to speed up the mass deportation of people in the country illegally.

It also marks one of the most sweeping assertions of federal authority over a local government in modern times.

While Washington has grappled with spikes in violence and visible homelessness, the city’s homicide rate ranks below those of several other major US cities and the capital is not in the throes of the public safety collapse the administration has portrayed.

A population already tense from days of ramp-up began seeing more significant shows of force across the city on Thursday.

US National Guard troops watched over some of the world’s most renowned landmarks and Humvees took position in front of the busy main train station.

Volunteers helped homeless people leave long-standing encampments – to where, exactly, was often unclear.

Department of Homeland Security police stood outside Nationals Park during a game on Thursday between the Washington Nationals and the Philadelphia Phillies.

DEA agents patrolled The Wharf, a popular nightlife area, while Secret Service officers were seen in the Foggy Bottom neighbourhood.

For an already wary Washington, Thursday marked a notable – and highly visible – uptick in presence from the previous two days.

The visibility of federal forces around the city, including in many high-traffic areas, was striking to residents going about their lives.

Trump has the power to take over federal law enforcement for 30 days before his actions must be reviewed by Congress, though he has said he’ll re-evaluate as that deadline approaches. (AP)