The Ultimate Fighting Championship (UFC) event at the White House is underway, after lightning tore across the evening sky as the first two fighters walked out into the ring.

The championship, the first ever professional sporting event held at the presidential residence, had been threatened by thunderstorms for much of the day โ€“ but went ahead despite initial delays.

It opened with US President Donald Trump walking from the White House to the arena along Dana White, chief executive of the UFC. Military planes then flew overhead.

About 4,300 people were expected at the invite-only mixed martial arts event on the South Lawn, with another 85,000 expected at a nearby fan zone.

A weather forecast that included the air thickening to humidity exceeding 70%, temperatures hovering around 80F (27C), and winds gusting through the area, meant the action was put off for an hour. Its original start time was 20:00 EDT (midnight GMT, 01:00 BST).

The event coincides with President Donald Trump's 80th birthday, which is also Flag Day, and is part of celebrations for America's 250th anniversary. The crowd sang happy birthday to the president.

The seven-bout card was arranged between Trump and his longstanding friend Dana White, UFC president.

Trump was seated directly next to the Octagon, near White and several members of the Trump family. Both his sons were in attendance. Other administration figures present included FBI Director Kash Patel, who was cheering loudly from his seat.

The fighters were escorted onto the stage by winners of the Medal of Honor - the highest award for battlefield actions - and first responders.

Some of the fighters acknowledged the uniqueness of the venue on stage.

"This is incredible," said Brazilian fighter Diego Lopes, the first man to win on the lawn.

The UFC spent about $60m (ยฃ45m) on the event, including $700,000 for grass repairs afterwards on a lawn that hosts the annual Easter Egg Roll.

The centrepiece is the "Claw" - a 92ft high metal structure that looms over the octagon and many of the seats. It weighs 600 tonnes.

A total of 14 fighters are competing in back-to-back fights, ending with the main card between Georgian-Spanish lightweight Ilia Topuria and American Justin Gaethje.

It is being screened exclusively on Paramount+, which is run by Trump ally David Ellison. Last year, UFC signed a $7.7bn deal with the Netflix rival streaming service.

The ultimate winner may be UFC - making it to the White House is a fist-pumping brand boost for a sport that was once shunned by sponsors and venues and denounced as "human cockfighting" by a US senator.

Administration officials - including Trump - have repeatedly praised the event, with the president referring to it as "the greatest show on Earth" and comparing the Claw to the Eiffel Tower in Paris.

Secretary of State Marco Rubio this week lauded the UFC as the "definition of American soft diplomatic power" and announced the launch of a private-public partnership to use the UFC as a diplomatic tool.

Just days before the fight, a lawsuit was filed on behalf of two Virginia residents - a Vietnam veteran and a local civic activist - to try halt the event.

The legal action, filed by the Public Integrity Project, a self-described anti-corruption law firm in Washington, argued the event was "deeply corrupt".

It cited a lack of approvals for the weigh-in at the nearby Lincoln Memorial and President Trump's close personal and financial ties with Dana White and the UFC.

But on Friday, a judge denied an emergency injunction to stop the fight - a ruling welcomed by the White House, which described the lawsuit as "frivolous".

A Reuters/Ipsos poll released on Friday, however, suggested many Americans were sceptical of the event.

The survey found that only 16% of Americans believed it appropriate to hold the UFC fight at the White House, compared to 46% who thought it inappropriate.

Only about a third of Republicans approved of the plan.

The UFC fight is one of several marquee events planned to mark the 250th anniversary of the country, which will also include an IndyCar race around the National Mall later this summer and a "Great American State Fair" in July.

It's was a hot and humid day on Sunday in the nation's capital, with mid-afternoon temperatures peaking at around 91F (33C), according to forecasts.

As the heat and humidity builds, scattered thunderstorms were expected to develop into the evening, with the risk of lightning, downpours and wind gusts of over 50mph (80km).

The summer humidity in Washington often attracts an unpleasant number of insects, and The Weather Channel is reporting that swarms of mosquitoes and gnats may find their way into the cage.

It also said that a single lightning strike within eight miles of the fight could trigger a 30-minute freeze on the event.

A news conference for the event at the Lincoln Memorial on Friday was briefly delayed over "inclement weather" and fans were urged to seek shelter.

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