US President Donald Trump will head to Washington DC's National Mall on Saturday for what he has billed as a "spectacular rally" celebrating America's 250th anniversary.

The event, taking place as a sweltering heatwave grips swathes of the eastern and central US, will include flyovers by hundreds of aircraft and a fireworks display organisers hope will be the biggest of all time.

Military flyovers over Washington DC will happen every hour between 13:15 local time (17:15 GMT) and sunset, the organisers said, and Trump's new Air Force One will feature in one of the formations over the capital.

The president, however, has been accused by opponents of politicising the nation's anniversary event and several music acts dropped out soon after being announced.

Extremely hot, humid temperatures of approximately 38C (100F) and a later-than-anticipated start time may also have an impact on the size of the crowd that attends.

There is also the potential for evening thunderstorms that could disrupt the events, which have been organised by a White House-backed public-private partnership.

President Trump used a speech on the eve of Independence Day to claim that American identity is under threat from what he called domestic "radicals" and "extremists".

Speaking at the the Mount Rushmore national monument in South Dakota on Friday, the president warned of a resurgence of what he called "the communist menace" and "newcomers" who embraced ideas opposed to the American way of life.

His comments reflect a growing political line of attack ahead of the November mid-term elections, with Republicans seeking to brand Democrats as communists.

The Washington DC event, formally known as the Salute to America 250 Celebrations & Fireworks - is due to begin at 19:00 local time, with Trump expected to speak a few hours later at approximately 21:45.

He has promised to make a "really long speech" at the Fourth of July festivities, despite the heatwave, "to show that I can do anything".

Other events that were scheduled for earlier on Saturday have been cancelled due to forecast high temperatures.

The Fourth of July fireworks display is an annual tradition in Washington DC and Trump has promised that this year's version - which is scheduled to begin after his remarks - would be the "largest fireworks show in history".

Approximately 850,000 fireworks will be launched from 10 sites across the city, including eight barges on the Potomac River, according to the event's organisers. It is expected to last for 40 minutes - twice as long as the usual display, which features about 20,000 fireworks.

It is aiming to secure the the Guinness World Record for the largest official fireworks show ever, surpassing the current record set in 2016 by a megachurch in the Philippines.

There are concerns, however, that the display could cause dangerous levels of air pollution in the city.

Internal National Park Service documents obtained earlier this week by outlets including the Washington Post and Politico said the fireworks were likely to cause "very unhealthy" conditions in the centre of the capital.

Soaring temperatures, potential storms and airport-style security checkpoints and could potentially put a dampener on the event, with administration officials reportedly concerned about the potential crowd-size.

The event has also courted political controversy in recent weeks.

The White House group organising it - Freedom250 - has been accused by critics of effectively supplanting the separate, bipartisan America250 commission created by Congress a decade ago, with some Democrats accusing Trump of hijacking the event - a charge organisers deny.

"President Trump couldn't help but try making America's 250th birthday all about himself," California Democratic Senator Alex Padilla said in a 24 June hearing.

Freedom250, for its part, has brushed aside those accusations, describing them as a "partisan smear" from politicians seeking to score "political points" rather than celebrate the US milestone.

Elsewhere in the country, the Times Square Ball in New York will drop eight times to signal midnight in every US time zone. There will also be a fireworks display in the city at 21:25 local time.

And in Philadelphia, where the Declaration of Independence was signed in Independence Hall 250 years ago, there will be a celebration concert featuring stars including Christina Aguilera and Meek Mill. The city is also hosting its final World Cup game earlier in the day. (BBC)

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