I haven't done one of these in a while. I found the 'past' photo on the excellent Portland Maine History facebook group page, within photo album #12. I could spend all day looking at all the old pictures of Portland that have been posted on the site.
Anyhow, this photo is on Congress Street. You will recognize the Porteous Mitchell and Braun Building on the left (now M.E.C.A.), the Foster-Avery's (men's clothing) / Clear Weave (hosiery store) building which is now home to Space Gallery and The Artist Studios, and the F.W. Woolworth's building on the right, which is now a Reny's.
Check out some of the other posts in my "looking into the past" series here or on my Flickr.