The grand military parade that US President Donald Trump had been wanting for years barrelled down Constitution Avenue on Saturday with tanks, troops and a 21-gun salute, playing out against a counterpoint of protests around the country by those who decried the US leader as a dictator and would-be king.

The Republican president, on his 79th birthday, sat on a special viewing stand south of the White House to watch the display of American military might, which began early and moved swiftly as light rain fell and clouds shrouded the Washington Monument. The procession, with more than 6,000 soldiers and 128 Army tanks, was one Trump tried to make happen in his first term after seeing such an event in Paris in 2017, but the plans never came together until the parade was added to an event recognising the Army's 250th anniversary.

“Every other country celebrates their victories. It’s about time America did too,” Trump declared in brief remarks at the parade's end.

Early in the evening's pageantry, the Army’s Golden Knights parachute team descended from overcast skies toward the reviewing stand. The team had been scheduled to appear at the end of the parade, but jumped earlier than planned in the drizzly skies above the National Mall.

At times, Trump stood and saluted as troops marched past the reviewing stand. But attendance appeared to fall far short of early predictions that as many as 200,000 people would attend the festival and parade. There were large gaps between viewers near the Washington Monument on a day when steamy weather and the threat of thunderstorms could have dampened turnout.

Hours before the parade started, demonstrators turned out in streets and parks around the nation to sound off against the Republican president. They criticised Trump for using the military to respond to people protesting his deportation efforts and for the muscular military show in the US capital.

The daylong display of America’s Army came as Trump has shown his willingness to use the nation’s military might in ways other US presidents have typically avoided. In the last week, he has activated the California National Guard over the governor’s objections and dispatched the US Marines to provide security during Los Angeles protests related to immigration raids, prompting a state lawsuit to stop the deployments. (AP)