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

At an extraordinary news conference on Monday, President Donald Trump revealed plans to take over the Washington, D.C. police department and deploy National Guard troops in the nation's capital to combat what he says it rampant violent crime that statistics show is actually decreasing.

Trump called federalizing the Metropolitan Police Department and deploying the National Guard to Washington "Liberation Day in D.C."