By the hundreds, they marched down America’s main Street Sunday evening to celebrate their own idea of freedom.

Even a looming summer storm forecast couldn’t dissuade thousands of Ultimate Fighting Championship fans from experiencing what was certainly a unique watch party for the pugilistic sport: Not only was it being held at the White House, but it coincided with President Trump’s 80th birthday.

In the hours before the first mixed martial arts bout were scheduled to begin in a newly built venue on the South Lawn of the White House, an army of lucky lottery winners navigated mazes of security barriers to watch the event on big screens in the shadow of the Washington Monument on the Ellipse. The viewing area was said to be capable of holding at least 75,000 people.

In addition to spectators for the Ultimate Fighting Championship, hundreds of District of Columbia police, federal agents and National Guardsmen flooded the National Mall along Constitution Avenue, blocking vehicle traffic for a square mile and effectively creating a security cordon around the White House.

As is typical for the area, protesters seized upon the occasion to make their points. A smattering of people carried signs deriding President Trump. There were also MAGA supporters, wearing their signature red hats. Other groups touted their religious beliefs or even their favorite fitness routine.

Carrying a bag emblazoned with “U.F.C. 250” slung over his shoulder, George Sanchez, 19, of Los Angeles, and his friend Jacob Diaz, 20, fast-walked past three Smithsonian museums in search of a way onto Constitution Avenue and the ticket entrance.

“How far back does this thing go?” Mr. Sanchez said to Diaz, as they searched for an entrance to the line.

Mr. Sanchez didn’t even remember entering the lottery to attend the watch party until he opened an email two months ago. He said he has been to seven or eight U.F.C. fight nights where he could watch the fights live.

The friends spent the first part of Sunday watching a World Cup soccer match on the National Mall in the shadow of Congress. By nightfall, they would be in the front the White House watching a combat sport akin to modern-day gladiator battles.

“I’ve been to a bunch of U.F.C. events, but you know, this one’s different, even though it’s not like actually seeing the actual fighters in the octagon, you know,” Mr. Sanchez said. “It’s still special to see. Like, you have the White House in the background, and everything, so it is super different.”

Kaan Sarican, a Las Vegas driver for Lyft who immigrated from Turkey, hoped to continue an incredible run of luck that brought him to Washington.

He landed in Nevada six months ago after winning a green card lottery. On Sunday, he was poised to experience his first U.F.C. event after winning a ticket in the viewing lottery. And just in case his good luck wasn’t over, he held a Powerball ticket in his phone case.

Mr. Sarican, 29, praised Dana White, the chief executive of U.F.C., and President Trump for pulling off an event that allowed people like him to attend for free.

“I’m proud of Dana White actually and Trump,” Sarican said. “We’re gonna celebrate the Trump’s birthday today.”

Sarican had another wish: A victory for his favorite fighter, Ilia Topuria.