(Portland, OR)  --  Police say a protest in Portland over the Ferguson grand jury decision resulted in traffic obstruction and arrests.  Authorities say the protest began peacefully yesterday afternoon at the Police Bureau's Central Precinct at Southwest Third and Madison, and the crowd began marching around downtown Portland at five p.m.  Police say some rocks were thrown during the march and a driver was assaulted.  Traffic was halted after marchers streamed onto I-5 and I-84, even though police in riot gear tried to block access.