User friendly transition between signup page and login page

I work on a website where users:

• fill in a registration form (with only 2 fields: password & confirm password; each user have a unique token which is stored in the URL)
• then land directly after on the login page (2 fields again: login & password)

During an UX session with end-users, we noticed that this is a bit confusing to them, because registration form and login form look very similar (two fields). Some users take some time to realize that they're actually on a different page and now need to log in.

In this context, what would be the best solution?

• Create a transition between both screens?
• Automatically log the user in after he sets his password? (This may be a security issue...)
• Somehow differentiate the screens? For example by putting some welcoming message on login page if user just registered...

Thanks in advance, I hope this post is appropriate, first time here.