"I'm not going to let some rain stop our 250th," Trump said in a social media post on Saturday.
"I will be there no matter what," Trump said.
"It's Saturday night, LETS HAVE SOME FUN, even if we are out late tonight."
Event organisers Freedom 250 said Trump planned to give his speech on the Mall at 11pm (1pm Sunday AEST) followed by fireworks, and gates for the Salute to America event would reopen at 9:45pm.
"Freedom 250 will share updates on programming and doors reopening," Freedom 250 spokesperson Danielle Alvarez said in a statement that encouraged participants to seek shelter at museums and federal buildings near the National Mall.
The US Secret Service said it was not letting any more people through security checkpoints.
Plans for fireworks were still moving forward in other cities including Chicago and New York, where tall ships passed the Statue of Liberty earlier in the day, recalling the fanfare around America's 200th anniversary in 1976.
Organisers of celebrations months in the making had to adjust or cancel activities entirely as much of the East Coast sweltered under heat that approached and in many cases surpassed triple digits.
The 250th anniversary of independence is colliding with a country gripped by political polarisation and a heat wave bearing down on millions of people across multiple states, forcing organizers of celebrations months to adjust or cancel activities entirely.
The signing of the US Declaration of Independence, one of history's most celebrated articulations of democratic ambitions, is being marked in myriad ways on Saturday.
Trump plans to speak at the National Mall is scheduled to come ahead of what is being billed as a historically enormous fireworks show that will rain down over the capital.
Hundreds of masked members of the white nationalist group Patriot Front marched through parts of the capital, ahead of Independence Day festivities planned for the evening that have been criticised as divisive.
The group posted on social media about 400 members had arrived in the capital, and Reuters photographers saw hundreds of people dressed in Patriot Front outfits travelling on DC Metro trains.
Videos posted on various social media platforms and shared on Patriot Front's own Telegram channel showed the group marching to drummers near the US Capitol and the Union Station transit centre while wearing khaki pants and caps, blue shirts, white face coverings and sunglasses.
Many were carrying the group's flag, Confederate flags, and variations of the US flag, at times chanting "Reclaim America".
Patriot Front, known for its uniforms, face masks and flash-mob-style demonstrations, formed in 2017 after the deadly "Unite the Right" rally in Charlottesville, Virginia,
The president was in South Dakota at Mount Rushmore on Friday, where he delivered a dark speech about the threat of communism in the US as the chiselled images of four of his most prominent predecessors loomed behind him.
New York hosted a ball drop at midnight to usher in the holiday with the same fanfare as New Year's Eve and tall ships paraded past the Statue of Liberty in a call back to the fanfare around the 200th anniversary in 1976.
Bristol, Rhode Island, describes itself as home of the country's oldest Independence Day celebrations dating to 1785.
In Los Angeles, Queen Latifah will host a concert featuring performances by The Smashing Pumpkins and Chris Stapleton.
Anticipation for the milestone holiday has been building for much of the year, serving as an opportunity for people across the US to reflect on their complicated history as one-time colonists of an empire who became a superpower of their own.
The country is celebrating 250 years since 13 colonies declared independence from Britain, triggering the Revolutionary War that ended with officials in London recognising US sovereignty in 1783.
Celebrations months in the making had to adjust or cancel activities entirely as much of the east coast sweltered under heat.
In Washington DC, the Great American State Fair was closed for several hours on Friday afternoon during the worst of the heat.
The city's main Independence Day parade scheduled for Saturday was cancelled but a smaller one rolled along in the Capitol Hill neighbourhood in the morning as onlookers sought shade under trees along the route.
To the chagrin of many on the east coast, the weather was downright pleasant on the other side of the country.
The Pacific Northwest enjoyed temperatures of about 20C on Friday with a few light showers.
The celebrations are unfolding against the backdrop of a deep divide this election year that has been expanding for years, visible in everything from political expression to cultural norms to age-old questions over race, class and immigration.
with Reuters
