On a red-white-and-blue day of remembrance and reflection, Sgt. John Flynn of the New York Police Department took advantage of free tickets to bring his wife and two young children to watch the Mets host the Chicago Cubs.

From coast to coast, American flags were unfurled across baseball fields, football gridirons and even a tennis court as fans of all ages sang the national anthem with gusto Sunday to mark the 10th anniversary of the Sept. 11 terrorist attacks.

Flynn said he had been a first responder at the World Trade Center a decade ago. Watching the ceremony at Citi Field, he got teary.

"How could you not?" he said. "It's just an amazing day, and it's a sad thing for everybody."

While baseball entered its final weeks and the U.S. Open tennis tournament crowned a women's champion, Sunday marked the start of the country's most popular sport: the NFL.

In choreographed presentations relayed to video screens around the league, "Taps" was played from Shanksville, Pa., where one of the hijacked jets crashed a decade earlier; Arlington National Cemetery in Virginia; and Hoboken, N.J., across the Hudson River from the World Trade Center site. A recorded message from actor Robert DeNiro was broadcast on videoboards reminding fans that "we honor those brave men and women by continuing to show our unity and strength as a country."

At halftime of the Jets-Cowboys Sunday night game in East Rutherford, N.J., the lights at MetLife Stadium were turned down for a 9/11 commemorative ceremony at halftime, with dozens of people gathering at midfield and forming two huge human squares _ in honor of the Twin Towers. Five For Fighting's John Ondrasik sat at a piano between the squares and performed his song, "Superman (It's Not Easy)," an anthem for many after the attacks with its lyrics about heroes.

"You wanted to win for yourself, but you wanted to win for the city," Jets safety Jim Leonhard said after New York rallied late for a 27-24 victory. "The city stepped up 10 years ago and this was really a celebration of what's gone on in the last 10 years. We were glad we could pull out a win. We kept fighting, just like they kept fighting."

NFL Commissioner Roger Goodell marked the day in Landover, Md., and East Rutherford.

"We remember our great country and the people that died in this tragic incident, the first responders and their families and all the people that kept our country safe," he told FOX from the sidelines of the Giants-Redskins game. "This is a chance for everyone to come together and feel great about our country, the sacrifices so many people have had and what we all have in front of us. We've got a lot to be proud of."

Reminders of the changes wrought by that sorrowful day were apparent outside MetLife Stadium, where every car entering the parking lots for the Cowboys-Jets game was checked by New Jersey State Troopers with bomb-sniffing dogs.

"It shows they're not taking any of this lightly," said Lee Loughridge from Mount Arlington, N.J. "I'm glad they're doing it. This is serious. We knew there would be delays, so we just came early and didn't have to worry."

Former President George W. Bush praised the rescue workers of that day in a televised pregame show segment prior to the openers, then did coin-toss honors at Cowboys-Jets. Yet another field-long flag was pulled across the turf as hundreds of hands reached out to touch it, including coaches and players. The skirl of bagpipes filled the air with "Amazing Grace," which, at times, was drowned out by fans' cheers of "U-S-A."

At the U.S. Open's women's final at Arthur Ashe Stadium, a "9/11/01" logo was painted next to the blue court, and Queen Latifah and the Jubilation Choir performed a soulful rendition of the anthem. The Marine Corps color guard unfurled a court-sized flag.

Pregame ceremonies were followed by moments of silence at Major League Baseball parks. There was a candlelight ceremony under dimmed lights at New York's Citi Field, but MLB denied a request by the Mets to wear caps honoring the NYPD and FDNY.

Mets players briefly considered defying MLB, then acquiesced.

"What are they going to do, fine us?" catcher Josh Thole said.

Joe Torre, MLB's executive vice president for baseball operations, told The Associated Press the decision was made to keep policy consistent throughout baseball and that "certainly, it's not a lack of respect."

At the Nationals game in Washington, two red, white and blue logos were painted on the field in foul territory along the base lines, with the date "September 11, 2001" and the words: "We shall not forget." The Nationals also wore blue jerseys with a stars-and-stripes background for the team's 'W' logo.

"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 fan from Bethesda, Md. "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."

At sun-splashed Soldier Field in Chicago, fans applauded the national anthem from start to finish _ a tradition at NHL Blackhawks games _ while tenor Jim Cornelison sang an unusually spirited anthem before the Bears hosted the Atlanta Falcons.

The ceremonies coincided with the regular-season return of the NFL following a summer of labor strife that threatened to stop play for the first time since 1987. The league planned to auction game-used items and donate $1 million to three memorials and two charities related to the attacks. The balls used for the kickoff of each half were inscribed with special 9-11 logos. Carolina's Steve Smith wore red-white-and-blue gloves and cleats for his team's game at Arizona, catching two touchdown passes in a 28-21 victory.

Jets players wore similar patriotic gear, and Cleveland Browns tight end Benjamin Watson donned red-white-and-blue gloves that read: "Never Forget."

Before the opening kickoffs, Bush appeared in a 90-second opening sequence of "Fox NFL Sunday" titled, "The Journey."

"What is a hero?" Bush asked. "In the midst of great danger ... nobody asks to be a hero. ... We started to heal with the help of our national pastimes, with the flip of the coin and the roar of the crowd."

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AP Sports Writers Greg Beacham, Rachel Cohen, Chris Duncan, R.B. Fallstrom, Howard Fendrich, Rick Gano, David Ginsburg, Mattias Karen, Mark Long, Janie McCauley, Richard Rosenblatt, Howie Rumberg, Dennis Waszak Jr., Joseph White, Barry Wilner and Tom Withers, Associated Press writer Gordon Brown, and AP freelance writer Daimon Eklund contributed to this report.