In addition to reducing threats, the N.F.L. hopes that the new clear-bag policy speeds up entry into stadiums, where lines are typically longest about 15 minutes before kickoff.

Teams notified their season-ticket holders of the changes this summer, and other fans learned about the changes through news media reports. Some, though, were caught unaware at the first preseason games this month.

Many accepted the changes as part of the league’s effort to remain vigilant. Carrying a see-through bag to the game was no worse than taking a laptop out of a knapsack in a security line at the airport, some said, as long as it was in the name of safety.

“I’d rather be safer than sorry,” said James Kenny, a retired police officer from New Jersey. “It’s almost like the airports with the different liquids. If you have nothing to hide, it’s fine.”

Not everyone views the enhanced security measures as benignly. Some fans suspect that the changes were aimed at preventing people from taking their own food and drinks into stadiums and forcing them to spend more at the concession stands. Women are upset that the policy prevents them from taking the purses they would normally carry and forces them to expose the contents in the clear-plastic bags.

“It’s discriminatory against women,” said Katie McSherry, whose thoughts were echoed by her sister, Claire, and their friend Alyssa Rossi. “Men don’t have to worry about it.”