In celebration of its 100th birthday, the bad old days have returned to Grand Central Terminal.

Droves of homeless people are flooding the landmark again — even as workers prepared for last week’s centennial celebration.

On a windy Wednesday, The Post spotted at least 40 hobos in search of free eats and a warm place to flop.

“It’s nauseating,” said Yvette Molina, a Tarrytown resident, who was lunching in the concourse. “It’s really disgusting. It makes you lose your appetite.”

In scenes reminiscent of the 1980s, one man picked through the garbage, rewarding himself with a half-eaten slice of pizza and a bag of nuts. A woman snored loudly as she slept, sitting upright on a concourse bench. And another man hoisted an oil can of Foster’s lager with filth-encrusted fingers.

The sights and smells — the stench of body odor easily overwhelmed the appetizing aromas from Magnolia Bakery — are enough to rattle the senses of even the most seasoned travelers and workers.

“Every day you see new homeless. Sometimes they smell — a bad smell,” said Hafiza Supti, who works at a Hudson News stand. “I have to move from one place to the other place.”

Police periodically roust the vagrants, but unless they’re closing the station for the night at around 2 a.m. or someone is breaking the law, cops won’t forcibly remove them.

“Our hands are tied,” said one MTA cop. “We can’t really make them do anything. We can’t just throw them in the cold.”

One of Grand Central’s denizens, James Patrick McGrath, 59, says cops are not always agreeable.

“I’ve got an officer who accuses me of begging,” the Flatbush native said through a matted beard. “He calls me a skell. I call him a schmuck.”

McGrath, who stared through rheumy blue eyes and was dressed in two ratty winter coats, said a few years ago, the homeless would seek shelter along the Metro-North tracks, but not anymore. Now, at night, they simply ride the rails if they are chased out.

“During the day, most people sleep at the tables,” McGrath said. “Forget about laying down — they’ll harass you.”

For decades since its opening on Feb. 2, 1913, the homeless used Grand Central as a place to crash till morning. But by the late 1980s, as the crack epidemic raged, the situation reached epic proportions, with 500 homeless staying there at any given time and at least 50 people living there year-round, according to the recent book “Grand Central: How a Train Station Transformed America.”

Now, the homeless increasingly take refuge there, and soft-hearted commuters can be part of the problem by offering handouts, say cops.

“People feel sorry for them,” he said. “We try to offer them services. Sometimes, they accept. Sometimes, they don’t. Most of them don’t want to be bothered.”

Harold Nelson, 58, counts himself among the latter category. He said he quit work as a hot-dog vendor because “there was no money in it.” He doesn’t want to live in a homeless shelter because of the rules.

Instead, he cycles through Grand Central, Penn Station and the Port Authority, seeking warmth during the colder months and spare change year-round.

Nelson says cops shuffle the homeless around during the day, but force them to leave between the closing hours of 2 and 5 a.m.

“The cops should be more considerate of people eating,” Molina said.