President Donald Trump's deployment of the 800-strong force has been described by Democrats as political theatre.
Five armoured carriers were parked near the Washington Monument with personnel accompanying the vehicle declining to comment, CNN reported.
A defence official told NBC News that troops will specifically be engaged in crowd management, perimeter control, security and communications support for law enforcement.
Washington Mayor Muriel Bowser sought to put a positive spin on the deployment, saying she wanted to use the extra personnel to drive down crime, despite earlier calling the move "unsettling and unprecedented."
She said the National Guard would not have power to arrest people.
Troops will carry no weapons but will have their standard issue firearms, usually rifles, close at hand, an official said.
In addition, Trump will send about 500 federal law enforcement agents to supplement the city's police force during the 30-day emergency deployment.
Earlier the White House said homeless people in Washington could face jail time if they do not comply with Trump's efforts to crack down on crime and rid the city of homeless encampments.
"Homeless individuals will be given the option to leave their encampment, to be taken to a homeless shelter, to be offered addiction or mental-health services, and if they refuse, they will be susceptible to fines or to jail time," White House spokeswoman Karoline Leavitt told reporters.
Leavitt said the administration was exploring strategies to relocate homeless individuals "far from the capital."
She said US Park Police have removed 70 homeless encampments from federal parks since March and are set to clear the remaining two encampments in the city later this week.
Andy Wassenich, director of policy at Miriam's Kitchen - an organisation offering services to the homeless - said his team was out trying to warn people.
He said there was still a lot of confusion about what the crackdown may bring.
Their best advice, he said, was: "Go to shelter if you can, if you can stand it. If you have anybody you can stay with, get off the street, and seek safety and let us know what we can do for you."
Trump described the homeless as one of several groups who have "overtaken" Washington that include "violent gangs and bloodthirsty criminals, roving mobs of wild youth, drugged-out maniacs."
He likened his intended crackdown to his administration's actions to secure the US border with Mexico.
US communities have long experienced seemingly intractable problems with homelessness, which reached an all-time national high of over 771,000 men, women and children on a single night in 2024, according to the Department of Housing and Urban Development's latest homelessness report to Congress.
The HUD report estimated Washington's homeless population at 5616, a 14.1 per cent increase from the year before. That made Washington, a city of just over 700,000 people, the 16th out of the 20 US cities with the largest homeless populations, according to the website USA Facts. The top five cities are New York, Los Angeles, Chicago, Seattle and Denver.
But the District of Columbia had the highest prevalence of homelessness among US states, with 83 homeless individuals for every 10,000 people, HUD data showed.
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