Washington DC - President Donald Trump will preside over a huge military parade in Washington on his 79th birthday Saturday, amid nationwide protests rejecting his authoritarianism.

But the threat of thunderstorms and the barrage of missiles raining down in Tel Aviv and Tehran – a conflict in which the US is a direct participant as Israel's main backer and sponsor – could cast a long shadow over the president's celebration.

"This is a big day for America!!!" Trump said on his Truth Social platform.

Nearly 7,000 troops plus dozens of tanks and helicopters will rumble through Washington in an event officially marking the 250th anniversary of the US Army, at a cost of up to $45 million.

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At the same time, protests by the "No Kings" movement are expected across the US against what organizers say is a Trump personality cult.

Organizers said the rallies would be the largest since Trump returned to office in January, adding that they were "rejecting authoritarianism, billionaire-first politics, and the militarization of our democracy."

Thousands more demonstrators are expected to turn out in Los Angeles to protest against Trump's deployment of troops in America's second-largest city following a mass uprising sparked by immigration raids.

Protests are set for other major cities including New York, Houston, Philadelphia, Seattle, Atlanta and Trump's second home in Palm Beach, Florida.

Organizers said they specifically left Washington off the list of cities holding rallies to draw attention away from the capital city, but dozens of veterans demonstrating against the parade in front of the Capitol were reportedly arrested Friday.

Trump says the Washington parade will be "like no other" but has also promised to use "very big force" if protesters attempt to disrupt his spectacle.

His violent language was echoed by other police forces in the country, including Sheriff Wayne Ivey of Brevard County, Florida, who threatened to kill demonstrators "graveyard dead."

The military parade is the largest in the US since the end of the Gulf War in 1991.

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Soldiers will wear uniforms dating back through history to its independence from Britain.

More than 50 helicopters will be involved including Apache gunships and Black Hawk transport choppers.

Around 150 military vehicles – including 28 Abrams battle tanks and 56 armored vehicles – will rumble past.

The parade is meant to end with a parachute display as members of the army's Golden Knights team jump in and present Trump with a US flag on Flag Day, which marks the adoption of the Stars and Stripes.

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

"No Kings" organizers said they expected millions of people in more than 1,500 cities to take part in the protests.

They said it was a "direct response to Donald Trump's self-aggrandizing" parade, "funded by taxpayers while millions are told there's no money."

An NBC News poll published Saturday revealed that 64% of Americans oppose using government funds for the military parade.

California's Governor Gavin Newsom, who slammed Trump for sending National Guard troops into Los Angeles without his consent, said it was a "vulgar display of weakness."

"To fete the Dear Leader on his birthday? What an embarrassment," Newsom, a Democrat, said Thursday, likening the display to a military parade in Kim Jong-un's North Korea.

In Los Angeles, where organizers said they expected 25,000 people to protest against Trump's immigration policies, there would be one huge balloon of Trump wearing a diaper and another of him wearing a Russian military uniform.