Protest organizers expected rallies in all 50 US states, calling them the largest since Trump returned to office in January, with the aim of "rejecting authoritarianism, billionaire-first politics, and the militarization of our democracy."

Wielding signs with messages like "No KKKings," "No crown for the clown" and "The Trump fascist regime must go now!" the protests stood in stark contrast to a massive military parade in Washington on Saturday.

The parade was meant to commemorate the founding of the US Army, but also fell on the president's 79th birthday.

"I am here today to tell the world that we don't have kings in America. In America, the law is king," Ilene Ryan told AFP at a demonstration in Boston.

In New York, tens of thousands of people, wearing raincoats and carrying colorful umbrellas, marched down Fifth Avenue in a downpour to the sounds of drums, bells and chants of "Hey hey, ho ho, Donald Trump has got to go!"

Actors Susan Sarandon and Mark Ruffalo were seen getting drenched among the protesters.

A few blocks away, Polly Shulman was preparing to join the march with her "Protect the Constitution" sign.

"I'm miserable and outraged about how this administration is destroying the ideals of the American Constitution," the 62-year-old museum employee told AFP.

The most shocking thing, she said, was "the illegal deportations of law-abiding residents."

They are "being kidnapped and disappeared and sent to torture prisons in foreign countries."

In March, the Trump administration expelled more than 250 Venezuelans to a mega-prison in El Salvador after accusing them of being members of the Tren de Aragua criminal gang, which it has declared a terrorist organization.

At least four protesters in New York were arrested at a separate, smaller protest against the Immigration and Customs Enforcement (ICE) agency, police said.

"I think people are mad as hell," said Lindsay Ross, a 28-year-old musician who urged others to show "the administration that we're not going to take this."

Bill Kennedy, a retired psychologist from Pennsylvania, was in Washington protesting a few hours before Trump's $45 million parade.

"I'm tired of the current administration. I think they're a bunch of fascists," he said, describing the military parade as "ridiculous."

Suzanne Brown in Boston also lamented the money spent on the parade "for one man's vanity."

Massive "No Kings" protests were  underway in Los Angeles, which in recent days has been rocked by demonstrations over the Trump administration's immigration crackdown, as federal agents swept up even law-abiding undocumented people. 

On Saturday, protesters shouted "You are not welcome here" at some of the 4,000 National Guard troops and 700 Marines that Trump dispatched to the city against the wishes of local and state authorities. 

With a giant orange Trump-in-a-diaper balloon towering above them, thousands filled the city streets, sporting slogans like "No faux-king way" and "Impeach Trump."

Members of Russian feminist protest and performance art group Pussy Riot held up a large banner in front of city hall warning: "It's beginning to look a lot like Russia." 

Iris Rodriguez, 44, explained that her family arrived in the United States without papers.

"I find it really, really personal... If this was my mom, if this were the 80s, this would be happening to her," she told AFP.

"I was a little scared, but I refuse to be too scared to not come."

The country-wide demonstrations overwhelmingly took place peacefully and without incident.

But in Culpeper, Virginia, police said a man "intentionally" rammed his car into a group of protesters as they left the event. No injuries were reported.

In Los Angeles, police used tear gas and mounted officers to clear protesters in front of the downtown federal building, the focus of anti-ICE demonstrations for the last week. 

AFP reporters said there was no disorder, but it appeared officers were moving people away from an area where National Guard troops and Marines were stationed.