Forecasters say there is a threat of lightning at the same time as nearly 6,000 troops, 50 helicopters and 150 armored vehicles are meant to roll through the capital, watched by thousands of spectators.

The Republican has been unlucky with the weather before, as freezing conditions meant that his inauguration for a second term in January had to be held indoors and a parade was canceled.

The White House vowed that a "historic celebration" of the US army's 250th birthday would go on even if there were "changes" due to the weather.

"Any changes to the Army Birthday Parade will be announced by the Department of Defense or America 250 Commission," White House spokeswoman Anna Kelly said in a statement to AFP.

"No matter what, a historic celebration of our military servicemembers will take place!"

The army and the America 250 Commission, which is responsible for arranging the parade, did not immediately respond to requests for comment.

Hundreds of anti-Trump "No Kings" rallies are planned on Saturday in cities across the United States -- although not in Washington -- in protest against what critics call the president's growing authoritarianism.

- 'Very big force' -

The rare military parade, the largest since the end of the Gulf War in 1991, comes after Trump sent National Guard troops and Marines to Los Angeles following protests.

Trump said on a visit to an army base on Tuesday that "we want to show off a little bit" with the parade, and vowed "very big force" if protesters try to disrupt it.

The army says the event could cost up to $45 million.

Nearly 7,000 soldiers will take part, wearing a variety of uniforms including some that date back through all of America's major wars since the Revolutionary War against Britain.

Roaring overhead will be more than 50 helicopters including Apache gunships, giant twin-rotor Chinook transport choppers and sleek Black Hawks.

Around 150 military vehicles -- including 28 M1A1 Abrams battle tanks, 28 Bradley Fighting Vehicles and 28 Stryker eight-wheel vehicles -- will rumble past too.

The route will take them past historic landmarks including the Lincoln Memorial and the Washington Monument, ending up near the White House.

The parade is meant to end with a parachute display as the army's Golden Knights team jump in and present Trump with a US flag.

- 'Believe in democracy' -

The event is being held on Flag Day, which celebrates the Stars and Stripes -- but it also coincides with Trump's own birthday.

Long fascinated with military pomp, Trump has openly envied the military spectacles seen in cities from Paris to Moscow and Pyongyang ever since his first term as president.

Trump has been particularly obsessed with having a parade since his first term as president when he attended France's annual Bastille Day parade in Paris at the invitation of president Emmanuel Macron in 2017.

"One of the greatest parades I have ever seen," Trump said shortly afterwards. "Because of what I witnessed, we may do something like that."

Back then he was put off by the huge cost -- an estimated $92 million -- and warnings that heavy tanks could damage Washington's streets.

But after his return to the White House in January, Trump would not be dissuaded again. This time, the army says metal plates will protect the roads from damage

Such displays of military might remain rare in the United States.

"We were founded by a group of merchants and farmers who were tired of a standing army invading their streets in the name of keeping them safe," Peter Loge, director of George Washington University's School of Media, told AFP.

"We've always looked down on grand military parades in Russia across Red Square or in North Korea, because we're not like that. We're Americans, and we believe in democracy, not in military shows of force."