As President Obama and 50 heads of state arrive for a weekend NATO summit, parts of Chicago are all but shutting down — the result of dire warnings about heightened security, snarled transportation and the threat of large protests downtown.

For weeks, the extensive preparations have been hard to miss. At one point, Black Hawk helicopters flying drills snaked between skyscrapers. Today, F-16 warplanes and other military aircraft will scream through the skies as part of a pre-summit defense exercise.

On the ground, barricades have been erected around landmark buildings. Many employees are being asked to work from home. And at least one apartment complex along a protest route advised residents to leave.

But others say some of the planning seems to be going overboard. A local columnist noted that in New York City, the short summit “would be just another day at the office.” And some Chicagoans are determined to keep to their plans and routines, including a bride who’s tracking the protests to make sure they don’t interfere with her wedding.

Matt Sweeney, a media planner for an advertising company, said his bosses required all workers to stay home Monday, the last day of the meeting.

“They’re worried it’s going to be really, really crazy,” Sweeney said. “I’m just glad they don’t have this summit every year. I’m thoroughly intimidated.”

Jonathan Vox, a patient-care consultant, said his company is letting employees work at home, too. But because he lives close to Wrigley Field, he has a second problem: On top of the demonstrations, his neighborhood will be swarmed by rival White Sox and Cubs fans arriving for a Crosstown Classic series, so he’s staying away from home as well.

“It’s an annoyance as much as anything,” Vox said. “It seems a bit weird that they’re all during the same weekend.”

The Metra rail system, which carries more than 150,000 Chicago-area commuters every day, is limiting the size of bags that can be brought on trains and prohibiting all food and beverages — even coffee.

Paul Filbin, a downtown lawyer, was headed to work Thursday from the suburbs, carrying a travel coffee cup and two shoulder bags, including one with his gym gear. He plans to work at home Monday, when he would be forbidden from bringing any of it.

“I can’t work out. I can’t bring my coffee. This may be brutal,” Filbin said.

The summit takes place Sunday and Monday at the McCormick Place Convention Center along Lake Michigan. Mayor Rahm Emanuel has said the security precautions will present a “minor” inconvenience while the summit helps showcase the city.

Writing in a Chicago Tribune column, John McCarron, an urban affairs expert, questioned breathless media reports about the NATO preparations, contrasting Chicago’s response with New York City’s routine handling of United Nations meetings.

New Yorkers, he said, would simply conclude they should “stay clear of East River Drive and the Midtown tunnel. But no, here in Third City, we’re acting like a bunch of bumpkins and nervous Nellies.”

Some precautions are prudent in the post-Sept. 11 era, McCarron said. But he lamented that no one bats an eye anymore when major institutions such as the city’s lakefront museums are shut down for security reasons.

“Has Chicago lost its ability to chew gum and walk?” he asked.

Still, downtown Chicago will look and feel different during the summit because of security measures large and small.

Illinois State Police and the National Guard will play a supporting role, along with police officers from Philadelphia, Milwaukee and Charlotte-Mecklenberg, N.C. Several highways and portions of Lake Shore Drive will be closed, and major delays are expected Monday when motorcades return to O’Hare International Airport.

However, companies in the Willis Tower, along with its tourist destination Skydeck, will remain open. “We’re planning for business as usual,” Tower spokeswoman Kate Murphy said.