At least five firefighters were injured. Three were evaluated at the scene and two taken to a hospital, an emergency medical services spokesman said.
A reporter for the Boston Globe said at least two people were removed from the building on stretchers.
The fire in the four-story brownstone on Beacon Street broke out while the city was whipped by high winds from a coastal storm.
Jack Mahoney, registrar at the nearby Newman School, said fans there were shut off because they were drawing smoke into the building. Visibility in the area was close to zero.
“Smoke is in the alleyways, it’s everywhere, it’s so musty in here," Mahoney said.