The theater, located at 45 E. 2nd St., caught fire shortly after 8:30 p.m., 10TV News reported.
There was a performance being held at the theater at the time of the fire. Witnesses said chunks of plaster were falling from the ceiling in the auditorium.
The building was evacuated. Several people were being treated for smoke inhalation.
The non-profit theater has been a site for performing arts and entertainment in the city since 1853, according to its Web site.
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