As noted on Mojang's site the Minecraft session servers are currently offline preventing players from logging in to any Minecraft server worldwide. We're tracking the status of the issue and will update this post as new information becomes available. This issue also causes 503 errors.

Please refrain from contacting ticket or live support about this issue - it is out of our control and we're trying to help as many users as possible.

Update: This issue is still occurring. Please keep an eye on the Mojang site for more information.