United States sports fans marked the 10th anniversary of the 9/11 terror attacks by unfurling giant American flags inside sold-out stadiums accompanied by racous chants of "USA! USA!"

The anniversary of the September 11 attacks on Washington and New York was also observed by pregame ceremonies and tearful moments of silence at every Major League Baseball and National Football League game.

MLB's Washington Nationals team painted a pair of red, white and blue logos on the field with the words: "We shall not forget" and the players wore blue jerseys with a prominent stars-and-stripes design.

"Frankly, I was a little bit skittish with regard to coming out to a ballpark and large gathering of people with feelings of how scared we were 10 years ago," said Joe Bailey, a 40-year-old American baseball fan.

"I think as part of our resolve, it's to go ahead and continue on in the American way and do what we do, and one of those things is to be passionate about baseball."

There was also a heart-felt rendition of the national anthem at an NFL game between the host Kansas City Chiefs and Buffalo Bills. Anthem singer David Cook had no sooner finished the song and a fleet of US airforce jets screeched overhead while the crowd barked "USA! USA!"

The military theme carried over into halftime of the game as the names of the 9/11 victims were displayed on the giant video scoreboard.

"This is a chance for everyone to come together and feel great about our country," NFL Commissioner Roger Goodell said.

Former US President George W. Bush, who launched the war on terror by invading Afghanistan and then Iraq, praised the rescue workers of that day 10 years ago in a televised pregame football show.

Washington gridiron fan Robin Berretta braved the crowd, and the fears of some of her friends, so she could watch her beloved Redskins in person.

"Everyone's very paranoid," Berretta said. "And they're not even from New York."

New York baseball star Jorge Posada wished he was in the Big Apple instead of on the road in southern California where the Yankees were playing the Anaheim Angels.

"I wish we were at home with the people of New York," said Posada.