For the beginning of his public debut, Bei Bei mostly snoozed on his back — a big puff of black-and-white fur with fluffy black ears curled in a box in a corner of his mother’s den at the National Zoo.

Crowds stood captivated, and a few fans even fought back tears, as the giant panda cub scooted on his side, curling his paws around his fuzzy belly. The napping cub was oblivious to his rock-star status.

But Bei Bei’s every little move prompted “pandamonium.”

“Bei Bei is just so cute, a little ball of fluff,” said Tess Wells, 24, a pharmaceutical student who traveled to Washington from North Carolina. Wells and her friends Shima Ghattab, 24, and Christie Duckett, 24, were among hundreds of panda lovers who began lining up outside the gates in the wee hours of Saturday before filing through the Asia Trail entrance.

Bei Bei, pronounced “bay bay,” was born Aug. 22. Until Saturday, the cub, named by first lady Michelle Obama and Chinese first lady Peng Liyuan, could be seen by most people only online through the lens of the zoo’s panda cam, where he has a significant following and generated millions of clicks.

Five-month-old giant panda cub Bei Bei makes his public debut in front of adoring fans at the National Zoo. (The Washington Post)

“There is something very special about giant pandas,” said Pamela Baker-Masson, director of communications for the Smithsonian’s National Zoo, who explained the public’s infatuation with the now 25-pound cub. “They are extremely rare. Their natural habitat is in China. There are only four zoos inside the United States where people can see them.”

Their distinctive black-and-white markings, she said, add naturally to the charisma of pandas.

“A baby bear is incredibly adorable,” Baker-Masson said. “Some of their behavior is interesting and mesmerizing. That is why people watch them on the panda cam — the way they sit up and eat bamboo. They are appealing for so many reasons.”

Giant pandas are an endangered species, threatened by humans. Only about 1,800 are left in the wild, according to the World Wildlife Fund.

Bei Bei, whose name means “precious treasure,” lives at the zoo with his 2-year-old sister, Bao Bao, whose name also means “precious treasure,” and his mother, Mei ­Xiang, whose name means “beautiful fragrance.”

Bei Bei’s older brother, Tai Shan, who was born July 9, 2005, and whose name means “peaceful mountain,” lives in China in a conservation center for pandas. (China owns and leases all giant pandas in U.S. zoos, and by agreement, any cubs born in the United States are sent to China when they are 4 years old.)

Bei Bei’s twin — who was never given a name — died when it was 4 days old after it inhaled some food product, which led to pneumonia.

Bei Bei’s father, Tian Tian, still lives at the zoo — although Bei Bei’s mother and father are separated. (Because giant pandas are solitary in the wild, they live apart, zoo officials explained.)

“They can see each other through a mesh ‘howdy window,’ if they choose,” zoo officials said.

Zookeepers explained how to distinguish Mei Xiang from Tian Tian. The mother, who weighs about 233 pounds, has black hip-high “stockings” extending up her hind legs. Tian Tian, who weighs about 264 pounds, has black “knee socks.” His name means “more and more.”

As people waited to enter the panda house, they were able to see panda mom, dad and sister outside. They watched Tian Tian roam his den and eat bamboo. They saw Bao Bao sitting up and mom Mei Xiang stripping bamboo.

Giant panda keeper Marty Dearie explained the differences in personality between brother Bei Bei and sister Bao Bao.

“Bao Bao was more independent at this age,” Dearie said. “She did what she wanted. Bei Bei seems to spend more time with his mother.”

Bei Bei is still taking milk from his mother. He has been nibbling on bamboo, although keepers are not sure whether he is actually consuming it, Dearie said. “He is chewing it, and it is mostly falling out of his mouth. He may have eaten a leaf, but we are still watching.”

Dearie said that the cub is typically awake when keepers arrive about 6:30 a.m. Then he falls asleep. He wakes again between noon and 2 p.m.

Friends of the National Zoo got special glimpses of the cub last week, but on Saturday all visitors could see Bei Bei. Anticipation grew as crowds inched closer to the panda house, where they were allowed to enter in groups of 50.

“I follow Bei Bei on Instagram,” Wells said. “When I saw they were debuting him this weekend, I sent my friend an immediate Facebook message. I said, ‘We’re going to see Bei Bei!’ ”

Wells and Ghattan, students in North Carolina, traveled to Washington to see Duckett, but mostly to see Bei Bei.

As the line moved closer to the panda house, Wells’s excitement grew. “This might get embarrassing,” she said, holding back her enthusiasm. “My mom and I watch Bei Bei on the panda cam, and we text each other if he is doing something really cute. I will watch for a while. If he’s asleep, I’m like, okay, he’s asleep.”

Even Ghattan, who admits she has an animal phobia, was excited: “I had a stuffed panda growing up as a child.”

Wells said that pandas “are so easy to love. Sometimes if there is a panda documentary on, showing that they can be ferocious, I won’t watch.”

Duckett added, “You don’t want to know the real truth.”

The line moved closer. “Oh, my God!” Duckett exclaimed.

“I will try to restrain myself,” Wells said. “If I go missing in the panda house, you will know why.”

Behind the girlfriends stood Christine Dillon, 34, an administrative assistant who lives in Fairfax, Va. Dillon traveled to the zoo with her mom, Jean Kelley, 56, a bank teller who lives in Woodbridge, Va.

“I love the pandas,” Kelley said. “I have followed them since we got our first set when Nixon was in office.”

Kelley was determined to witness Bei Bei’s debut.

“My husband knew I was coming today. There was no keeping me away. Even if there was a snowstorm, we were coming by hook or by crook.”

Inside the panda house, Bei Bei lay curled on his side in a box. Wells exclaimed, “Holy cow! He’s stirring a little bit.”

“I’m overwhelmed by his cuteness,” Duckett said.

Then Mei Xiang came into the panda house, climbed onto a pile of rocks and began eating bamboo. The cub looked up at his mom, scratched his neck with a fluffy paw, turned over and fell back asleep.