For those concerned: we were not in the Jamaa el-Fna square when the explosion occurred, although I walked by there just last night with one of our friends. Current reports indicate that this was an accident involving a gas tank.
UPDATE: Our apartment is just outside the square. I'm kind of surprised that we didn't hear the explosions, although we heard the sirens afterwards. We actually learned about the accident when #Marrakech became a trending topic on Twitter.
[UPDATE 2, 4:20 pm: News sources are now reporting that this was a bomb attack, not an accident.]
Here's the scene as of an hour ago.
As I'd mentioned, I was walking by the Argana last night with a friend. You can see a little bit of the building in the bottom left of the first picture, and its facade in the photo below that.