WASHINGTON : US President Donald Trump hailed America on its 250th birthday Saturday as the "crowning achievement" of human history, even as he used the event to renew his attack on domestic opponents he branded communists.

In a speech delayed by several hours when storms forced the temporary evacuation of crowds in Washington, Trump claimed that under his presidency the United States was "prouder than ever before."

While Trump had promised a huge political rally to stamp his brand on the national celebrations, the 80-year-old Republican largely stuck to a more traditionally patriotic script.

"For two and a half centuries, our American republic has stood as the crowning achievement of human history," Trump told tens of thousands of people on the National Mall.

On stage, he hailed also veterans from World War II and the Korean and Vietnam wars.

But he then hailed the latter two as examples of the battle against "communists" -- reprising his message from his speech on Friday night at the iconic Mount Rushmore monument.

"Our warriors did not fight communism on battlefields across the word, only to have that menace rear its ugly head right back here in America. We're not going to let it happen," he said.

- 'Proud to be American' -

Trump has repeatedly hammered home the theme recently ahead of November's US midterm elections, after the anti-establishment left of the Democratic Party won a string of US primary victories.

"It's like a cancer, you've got to cut it out," he added.

The US leader also used the speech to boast of the recent military campaigns against Iran and Venezuela, saying Washington had "wiped out" Tehran's military.

But the speech was short by Trump's standards, at around 45 minutes long.

"We love Trump, we loved his speech," said Richard Sullivan, 70, who came from Virginia with his wife Nancy and was wearing a Freedom 250 T-shirt.

"He went through our great history and had a word for the veterans that were here. He makes us proud to be American."

A huge fireworks display began after the speech. Trump has said it will set a record as the world's largest.

Americans braved a scorching heatwave in cities across the east of the country to celebrate the anniversary of the signing of the Declaration of Independence from Britain in 1776.

Temperatures soared to a record 103F (39.4C) in the capital -- an all-time high for July 4 -- with 160 million Americans under extreme weather warnings, according to the National Weather Service.

Inclement weather meanwhile forced fireworks in New York to be pushed earlier, and resulted in evacuations at a concert in Philadelphia and a celebration on the Charles River in Boston.

- 'Awesome country' -

The elements also threatened to wreak havoc with Trump's speech as tens of thousands were ordered to clear the National Mall several hours before his address due to approaching thunderstorms.

While many streamed toward exits, chaos broke out as other attendees refused to leave or tried to surge back in, where AFP reporters witnessed shouts of "charge!" and "Trump! Trump!"

Trump however insisted he was pushing ahead with the speech, telling Fox News that if D-Day veterans in World War II could brave bad weather then so could he.

"Experiencing a little heat is a lot less than what a lot of people sacrifice to give us this freedom in this awesome country," said Randy Cole, 62, a retired civil servant attending festivities in Washington.

Yet Trump's determination to put himself and his political brand front and center of the 250th anniversary celebration had highlighted the deep divisions caused by his second term in office.

Those divisions were on full display Saturday near Washington's Capitol Hill, where masked men -- some of them carrying Confederate flags and others sporting logos of the white supremacist Patriot Front -- gathered to shout, "Reclaim America!"

For Americans, the 250th anniversary offers a moment for reflection as well as celebration.

A Quinnipiac University Poll showed 61 percent of Americans thought the US was not living up to the ideals stated in the Declaration of Independence.