President Donald Trump sang the praises of the US military and American heroes of the past two and a half centuries Thursday, skirting politics in a rousing Independence Day speech in Washington.

"What a great country," Trump exclaimed in an address saturated with patriotism and exceptionalism, after critics accused him of hijacking the annual celebration.

"For Americans, nothing is impossible," he said.

Combat aircraft, including the rarely-seen B2 stealth bomber, flew overhead as Trump scrolled through myriad events of US history, from groundbreaking inventions to battlefield victories. He drew cheers of "USA! USA!" from an enthusiastic rain-soaked audience of tens of thousands on the National Mall in Washington.

Trump promised the United States would soon again send men to the moon, and go beyond to "plant the American flag on Mars."

"We will always be the people who defeated a tyrant, crossed a continent, harnessed science, took to the skies, and soared into the heavens, because we will never forget that we are Americans, and the future belongs to us."

'The most exceptional nation'

Trump spoke in front of a massive statue of a seated Abraham Lincoln, the heroic 19th century president, where civil rights icon Martin Luther King Jr gave his "I Have a Dream" speech in 1963.

Trump used the traditionally politics-free holiday to deliver shoutouts to each arm of the military, as well as singling out first responders and the controversial Border Patrol and Immigration and Customs Enforcement agencies that have been criticised for their treatment of migrants.

But he disappointed critics who had warned that Trump, the first president in decades to make a keynote speech on the July 4 holiday, was hijacking the celebration to bolster his own political standing and attack Democratic rivals.

Instead, in a trickling rain, he uncharacteristically avoided talking about himself and his political detractors. He repeatedly ascribed a singular greatness to the country, declaring it "the most exceptional nation in the history of the world."

"Today, we come together as one nation with this very special Salute to America. We celebrate our history, our people, and the heroes who proudly defend our flag -- the brave men and women of the United States military!" he said.

"Our nation is stronger today than it ever was before. It is its strongest now," he said.

Patriotic fervor

The July 4 celebrations usually draw hundreds of thousands of people to the National Mall for picnics and twilight fireworks, and dousings from thunderstorms did not keep away either Trump's supporters -- wearing red caps -- nor protesters.

"It's going to be fabulous, I'm excited," said Dee Ranson, a 55-year-old from Florida visiting with her family.

"I think it's awesome, I think it shows courage and patriotic enthusiasm. He is not afraid," she said before Trump's speech.

But April Smith, 43, visiting from North Carolina, said she had "no interest" in hearing Trump's speech, and thought the military "show of power and strength" was unnecessary.

Surrounded by the top officers of the Pentagon, and a panoply of invited Republican officials and VIP donors, the event allowed Trump to stake a strong claim to patriotic fervor 16 months ahead of a presidential election.

Polls show his potential Democratic rivals holding a significant edge over him.

Trump originally wanted a grand military parade for the holiday, ostensibly inspired by France's rollout of its armed forces on its own national day.

Even so, it was a show of martial force that has been absent from the US capital for decades, and Trump's foes blasted it ahead of time as a show of militarism.

"What, I wonder, will Donald Trump say this evening when he speaks to the nation at an event designed more to stroke his ego than celebrate American ideals?" leading Democratic White House contender Joe Biden said earlier Thursday.

Trump himself revelled in the strong crowd, having spent the past 30 months defending himself over the poor turnout for his January 2017 inauguration.

"A great crowd of tremendous Patriots this evening, all the way back to the Washington Monument!" he tweeted, along with a photograph of the audience.