How can I break a mesh into pieces? I've puchased a big interior scene from a 3d marketplace. 

To optimize it in Unity (using Occlusion culling), I need to break the meshes into pieces, without modifying the structure. 

Is there any blender command to do this?
In Edit Mode you can mark the polygons you want to separate and press P , then Separate by Selection.

Pressing L while hovering over a mesh in Edit Mode will select connected mesh parts.