Balcony or Roof Collapses in London's West End

Just happened a few minutes ago--at theater (where I attended show three years ago) during a performance and numerous casualties reported.  "Eyewitnesses said they saw people being escorted out of the building, covered in dust and debris."  Details as they emerge...See tweets from the scene.  And live TV coverage. Seems to be divergent reports that the collapse involved the balcony--or the roof.  One tweet from someone in audience seemed to suggest maybe could be worse--just front of balcony came down, not whole thing..."I think the front part of the balcony fell down. At first we thought it was part of the show. Then I got hit on the head."  Update:  Over 700 were in theater but none trapped now.  Estimates of injured range from 40 to 70, no confirmed deaths so far.