Trump hailed the United States 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 teargas to disperse protesters in Los Angeles and US ally Israel traded missile fire with Iran in a rapidly escalating conflict in the Middle East.

Trump's parade on an overcast night in Washington came after hundreds of thousands of "No Kings" demonstrators thronged the streets in cities including New York, Philadelphia, Houston and Atlanta.

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."

The display of military might comes as Trump asserts his power domestically and on the international stage.

Trump used his parade address to send a warning to Washington's adversaries of "total and complete" defeat, with the United States increasingly at risk of entanglement in Israel's conflict with Iran.

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

- 'Happy Birthday' -

Trump had openly dreamed since his first term as president of having a grand military parade of the type more often seen in Moscow or Pyongyang.

The last such parade in the United States was at the end of the Gulf War in 1991.

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.

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 than one of the barnstorming rallies that swept Trump to power.

The White House said that "over 250,000 patriots showed up" for the event, without providing evidence. Communications Director Steven Cheung described the "No Kings" protests as a "complete and utter failure."

"No Kings" organizers said protesters gathered in hundreds of places, with AFP journalists seeing large crowds in several cities.

Organizers said they were protesting against Trump's dictatorial overreach, particularly what they described as the strongman symbolism of the parade.

"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.

Some protesters targeted Trump's Mar-a-Lago estate in Palm Beach, Florida, while a small group even gathered in Paris.

- 'Display of authoritarianism' -

"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 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 (0300 GMT) curfew.

Police officials said a "small group of agitators" had begun throwing rocks, bottles and fireworks at officers, prompting the decision to deploy tear gas and order the crowd to disperse.

Violence shattered the calm elsewhere, with a shooting at a "No Kings" demonstration in the western US city of Salt Lake City.

At least one person was critically injured and three people were taken into custody, police said.

The killing of a Democratic lawmaker and her husband in the northern state of Minnesota on Saturday, in what the governor called a targeted attack, also cast a pall over the parade.

Trump was quick to condemn the attacks outside Minneapolis in which former state speaker Melissa Hortman and her husband were killed, while another state lawmaker and his wife were hospitalized with gunshot wounds.