Washington DC - President Donald Trump hosted a largely lackluster US military parade on his 79th birthday as protesters rallied across the country against his authoritarian rule.

Trump hailed the US as the "hottest country in the world" after watching tanks, aircraft, and troops file past him in Washington on Saturday to honor the 250th anniversary of the US army.

It formed a stark split screen with turmoil at home and abroad, as police used tear gas to disperse protesters in Los Angeles and Israel launched a US-backed assault on Iran.

The Republican largely avoided his usual domestic political diatribes in an unusually brief speech and instead focused on praising the US army, saying that they "fight, fight, fight, and they win, win, win."

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Trump used his parade address to send a warning to Washington's adversaries of "total and complete" defeat.

"Time and again, America's enemies have learned that if you threaten the American people, our soldiers are coming for you," he said.

Trump stood and saluted on a stage outside the White House as tanks rumbled past, aircraft roared overhead and nearly 7,000 troops marched by, in what observers criticized as a poorly organized and ramshackle effort.

Troops and military hardware from different eras passed by, with an announcer reeling off US victories in battles with Japanese, German, Chinese, and Vietnamese forces in past wars.

The army said the parade cost up to $45 million.

The crowd sang "Happy Birthday" and there were occasional chants of "USA! USA!" but the atmosphere was less intense and well-attended than the rallies that swept Trump to power.

The White House said that "over 250,000 patriots showed up" for the event, without providing any evidence.

Meanwhile, organizers of the "No Kings" movement said protesters gathered in hundreds of places, with total attendance reported at over five million.

"I think people are mad as hell," Lindsay Ross, a 28-year-old musician, told AFP in New York, where tens of thousands of people rallied.

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"I think it's disgusting," protester Sarah Hargrave (42) said in the Washington suburb of Bethesda, describing Trump's parade as a "display of authoritarianism."

Thousands more turned out in Los Angeles to protest against Trump's deployment of troops in the country's second-largest city following clashes sparked by immigration raids.

"He's trying to bully Los Angeles into complying with everything that he's trying to do, and we're not going to do that. We're a city of immigrants," a protester who gave his name as Armando told AFP.

After a day of largely peaceful protests, police unexpectedly began moving people away from the LA protest area, igniting confusion and anger among demonstrators caught off guard and unsure of where to go.

Police on horseback pushed crowds back as law enforcement fired tear gas and flash-bang grenades hours ahead of an 8:00 PM curfew.

Violence shattered the calm elsewhere, with a shooting at a "No Kings" demonstration in Salt Lake City killing one person, and a man driving into a group of protesters in Northern Virginia.

Florida Governor Ron DeSantis had previously encouraged drivers to mow down demonstrators if they feel threatened.