US President Donald Trump has posted an astonishing AI-generated video of him as a fighter pilot dumping brown liquid on American protesters.

The video, which Trump posted online late on Saturday (local time), shows him wearing a crown as Kenny Loggins’s song Danger Zone plays in the background.

The jet, emblazoned with “King Trump”, flies over Time Square in New York City, where Trump drops what appears to be sewage on the marching protesters below.

It came hours after up to seven million people marched and rallied across the country for “No Kings” demonstrations, decrying what participants see as the US government’s swift drift into authoritarianism under  Trump.

Organisers said there were peaceful demonstrations in 2700 cities and municipalities on Saturday – hundreds more locations than in a previous protest in June.

People carrying signs with slogans such as “Nothing is more patriotic than protesting” or “Resist Fascism” packed into Times Square and rallied by the thousands in parks in Boston, Atlanta and Chicago.

Demonstrators marched through Washington and downtown Los Angeles and picketed outside capitols in several Republican-led states, a courthouse in Billings, Montana, and at hundreds of smaller public spaces.

The Republican Party disparaged the demonstrations as “Hate America” rallies, but in many places the events looked more like a street party.

There were marching bands, huge banners with the US Constitution’s “We The People” preamble that people could sign, and demonstrators wearing inflatable costumes.

It was the third mass mobilisation since Trump’s return to the White House. It came against the backdrop of a government shutdown that has closed federal programs and services and is testing the core balance of power, as an aggressive executive confronts Congress and the courts in ways that protest organisers warn are a slide toward authoritarianism.

In Washington, Iraq War Marine veteran Shawn Howard said he had never protested before but was motivated to show up because of what he considered the Trump administration’s “disregard for the law”.

He said immigration detentions without due process and deployments of troops in US cities were “un-American” and alarming signs of eroding democracy.

Trump, meanwhile, spent the weekend at his Mar-a-Lago home in Florida.

“They say they’re referring to me as a king. I’m not a king,” he said in a Fox News interview that aired on Friday, before he departed for a $US1 million ($A1.5 million)-a-plate MAGA Inc fundraiser at his club.

As well as the AI video, the White House posted a separate image on social media showing Trump and Vice President JD Vance wearing crowns, juxtaposed with House Minority Leader Hakeem Jeffries and Senate Minority Leader Chuck Schumer wearing sombreros.

“We’re built different,” the caption read.

In San Francisco hundreds of people spelled out “No King!” and other phrases with their bodies on Ocean Beach. Hayley Wingard, who was dressed as the Statue of Liberty, said she too had never been to a protest before. Only recently she began to view Trump as a “dictator”.

About 3500 people gathered in Salt Lake City outside the Utah State Capitol to share messages of hope and healing after a protester was fatally shot during the city’s first “No Kings” march in June.

And more than 1500 people gathered in Birmingham, Alabama, evoking and the city’s history of protests and the critical role it played in the Civil Rights Movement two generations ago.

“It just feels like we’re living in an America that I don’t recognise,” said Jessica Yother, a mother of four. She and other protesters said they felt camaraderie by gathering in a state where Trump won nearly 65 per cent of the vote last November.

While protests earlier this year – against Elon Musk’s cuts and Trump’s military parade – drew crowds, organisers say this one united the opposition.

The national march against Trump and Musk this spring had 1300 registered locations, while the first “No Kings” day in June registered 2100.

“We’re here because we love America,” Democrat Senator Bernie Sanders said, addressing the crowd from a stage in Washington. He said the American experiment was “in danger” under Trump but insisted, “We the people will rule”.

Republicans sought to portray protesters as far outside the mainstream and a prime reason for the government shutdown, now in its 18th day.

From the White House to Capitol Hill, GOP leaders called them “communists” and “Marxists”.

“I encourage you to watch – we call it the Hate America rally – that will happen Saturday,” said House Speaker Mike Johnson of Louisiana.

-with AAP