The basic scene set up is this:
At a Farmer's Market in Coast City (the fictional hometown of our hero, Hal Jordan), citizens are terrorized by an unknown entity that begins to sow destruction.
The extras have been spending their days running around the set screaming and acting terrified. The Farmer's Market was constructed from scratch in a New Orleans parking lot. Ryan Reynolds hasn't appeared on set yet, but this scene does involve his character facing off against the threat. This threat could be the fear entity known as Parallax, since the other villain, Hector Hammond (played by Peter Sarsgaard) hasn't been confirmed to appear in the scene.
The loudspeaker voice you hear is the film's director, Martin Campbell, who has recently returned to the New Orleans set from a brief visit to last week's San Diego Comic Con:
UPDATE: Birds-eye-view photos of the set courtesy of DogsofWar (thanks to scooper Greg)
[Click on the images to enlarge]