A portion of a parking garage being built to house buses in downtown Los Angeles collapsed on Friday, just minutes after nearly 200 construction workers were alerted that something was wrong and fled to safety, officials said. A section of the structure measuring about 50 by 75 feet came crashing down just before 9 a.m. in the heavily congested area. Minutes earlier, 180 people had been working on the structure, said Metropolitan Transportation Authority spokesman Marc Littman.