If you were at the Boston Public Library on Tuesday and thought you heard a collective “Om’’ emanating from a room upstairs, you weren’t crazy.

More than 50 people packed into the Copley Square branch’s historic Abbey Room for the first of six free public yoga classes.

Led by instructor Rebecca Pacheco, the free workshop allowed locals to practice their sun salutations in a stately room with soaring, ornate ceilings and colorful murals depicting Sir Galahad’s quest for the Holy Grail.

“This room is so beautiful,’’ said Megan Fields, a 25-year-old Boston resident, as she filed out of the class. “When I got in, I laid down on my back and just looked up at the ceiling — it was gorgeous. If you go to a studio, you just get a white plaster ceiling.’’

The free class also drew some local celebrities. Susan Tran, a reporter for WHDH-TV (Channel 7), said she just started doing yoga, and had jumped at the chance to work on her moves in the one-of-a-kind space.

“This library is so gorgeous and stunning,’’ she said, gesturing around her. “To be able to do yoga and have an hour of practice here was really exciting. I put it on my calendar as soon as I could.’’

Pacheco, who led the class, is a bit of a local celebrity herself. She is a yoga expert for “Runner’s World’’ magazine, and last March published a book through HarperCollins called “Do Your Om Thing.’’ The book has sold more than 13,000 copies, she said. Several attendees lined up after class to buy a signed hardcover copy.

“I actually wrote some of the book in Bates Hall,’’ Pacheco said, referring to the iconic room next door to the Abbey Room that features rows of green reading lamps. “So it felt really special to merge those two worlds and teach in the library.’’

One yogi said he took a half-day off from work specifically to travel to the library for the class — all because he had read Pacheco’s book and jumped at the chance to take a lesson with her.

“It was a good class and Rebecca had good energy,’’ said Ryan Andrews, a 34-year-old from Arlington who had never visited the library before. “To do yoga my first time being here was kind of unique.’’

Rebecca Campbell, the librarian who organized the program, said other branches throughout the city offer similar, smaller events, but Tuesday’s class was the first time ever in the Abbey Room, a space that can fit about 150 seats and is also popular for weddings and lectures.

The next five classes will occur on Tuesdays at noon over the next few weeks, though the event may be moved to a larger room due to surging interest, said library spokeswoman Rosemary Lavery.

Pacheco, a 36-year-old South End resident, said that she has traveled the world in her yoga pursuits, but teaching open classes in the city where she lives feels particularly special.

“This is my home,’’ she said, adding, “You really can’t find a yoga studio that comes close in terms of beauty and atmosphere.’’

Jaclyn Reiss can be reached at Jaclyn.Reiss@globe.com. Follow her on Twitter @JaclynReiss.