The US president has declared a public safety emergency as he promises to tackle crime and homelessness in the nation's capital.

U.S. President Donald Trump said on Monday he was deploying 800 National Guard troops to Washington and temporarily taking over the city's police department, an extraordinary assertion of presidential power in the nation's capital.

The White House increased the presence of federal law enforcement in Washington over the weekend, deploying 450 officers across all four quadrants of the city Saturday and on Sunday in high-traffic areas, as Trump has ramped up threats of taking control of D.C. and its local police force.