“We’ve had our 9/11. They had what happened to them. We're all one, without question. We sympathise absolutely,” said Craig Corcoran of the Bronx, referring to the September 11, 2001, attack on New York’s World Trade Center.

“It's not going to stop us from having a good time. Over there, it's the same way. They're still going to go out and have their fun and they should,” Corcoran said while tailgating in the parking lot outside of MetLife Stadium in East Rutherford, New Jersey.

While the U.S. Department of Homeland Security informed the National Football League there were no known specific threats, Commissioner Roger Goodell was taking no chances and ordered beefed-up security at all venues.

“The safety of our fans, stadium personnel, and teams at all NFL games is our priority, and security at our games is robust,” the league said.

The NFL and other North American professional sport leagues went on high alert after one of the attacks on Friday targeted a friendly soccer match between France and Germany where suicide bombers apparently attempted to enter the Stade de France, in a suburb just north of Paris.

At Baltimore's MT&T Bank Stadium, K-9 units were visible outside the venue and inside around the concourse for the Ravens' game against the Jacksonville Jaguars, ESPN reported.

In Green Bay, Wisconsin, gate security was tight for the Packers' game against the Detroit Lions.

“They checked everything we had twice before they let us in,” said Dave Steavpack, who has been going to games at Lambeau Field for nearly 30 years. “They're obviously taking no chances.”

At MetLife, where the New York Giants were hosting the unbeaten, reigning Super Bowl champion New England Patriots in one of Sunday's marquee games, trucks and vans got special attention at the stadium complex.

“We've been coming here for 17 years but this time they were stopping all the pick-up trucks and some of the vans coming in because of what happened in Paris,” said Tony, who along with Bob and Rocco had come in early from Connecticut to tailgate in the stadium parking lot before the game.

“If they had the thing (cargo area) covered, they uncovered it. If they had boxes, they had to open up the boxes. This never happened before.”

Bob noted another first. “It's the first time I saw bomb-sniffing dogs,” he said.

“They were checking inside every one of those pots,” he said about the line of concrete planters that served as barriers ringing the perimeter of the entry gates.

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In addition to the NFL's routine metal detector screening at its games there were multiple layers of perimeter security employed including State troopers, who were out in full force at entry gates and in the parking lots to provide a watchful eye.

Among special precautions was a complete ban on entering stadiums with backpacks, satchels or purses. Fans could only carry in small, clear plastic bags.

A moment of silence was observed at Sunday's games with the French flag shown on video boards, and in some stadiums, including MetLife, the French flag was also exhibited by national colour guard details during the anthem.