A section of masonry 12 feet high and 4 feet wide came loose from the 40th floor of an office building on Lexington Avenue Monday morning, showering the crowded midtown street below with thousands of pieces of brick, two of which pierced the roof of a commuter bus.

No one was injured in the accident, which officials said appeared to have occurred when badly rusted metal ties that held the bricks in place broke as two workers used grinding tools to perform a waterproofing job one floor below on a hanging platform.But the collapse at 10:19 a.m. caused moments of terror and panic on Lexington Avenue between 46th and 47th streets, as pedestrians scrambled for safety, some darting into traffic to get away from the building, witnesses said. Later, as city safety officials moved in, cordoning off a four-block area, the street became a deserted scene all too familiar after a series of recent collapses around midtown: businesses closed, frustrated office workers shut out of their buildings and angry drivers trying to find a way around.

City officials said Monday that the sidewalk below had not been sheltered with a sidewalk shed, of the kind that now cover dozens of walkways around city construction sites, because the work being performed at the building was simply maintenance.