There were no injuries and little damage from the third-floor explosion, which startled library patrons around 2:30 p.m., said police spokesman Joe Cyr.
"Louder than any gunshot I've ever heard," said library patron Austin Carlson, who was near the device when it went off. "It smelled like natural gas."
Library board member Roger Sandack said the bomb was detonated from a chair near the fireplaces at the west end. Little damage was done to the chair but the blast destroyed a west-facing window that overlooks 200 East. No books or other materials were damaged.
Cyr said the explosion appeared to be a homemade device. One person was briefly detained, but police had no immediate suspects, he said.
Eastbound traffic was closed on 400 South at 200 East as about 400 patrons were escorted across the street to Washington Square.
Korey Llewelyn, a library patron who said she was in the basement in the children's section when the explosion detonated, said she felt a jarring sensation.
"I just thought somebody knocked a whole shelf over," Llewelyn said.
The explosion occurred as the Utah Historical Society was holding its annual meeting at the library.
Susan Whetstone, the society's photo curator, said the detonation was louder than a gunshot. "It sounded like an explosion."
Philip F. Notarianni, the society's director, said the explosion sounded like a gunshot, and said as meeting attendees were evacuating the library, "it smelled like gun powder."
But Notarianni said he could not see smoke or other signs of an explosion.
About 250 people were attending meeting activities in the library's basement and first-floor auditoriums, he said.
The incident forced the early closure of the city's reading palace and top tourist attraction - normally open until 6 p.m. on Fridays.
"But we'll be open tomorrow for business as usual," Sandack said.
The Salt Lake City Main Library was designed after the Oklahoma City and Sept. 11 tragedies - its structure and layout employ all the latest technologies to ensure patron safety, Sandack said.
"We're safe. We have had in the library for years policies and procedures for these kinds of emergencies . . . the staff are very well-schooled in what to do and how to protect everything," said Sandack.