MacBook pro: disk space is full - can't access anything!

I'm running a MacBook pro 13" and I'm having a problem
never previously encountered.
I turn on the computer and I arrive at the login screen.
When the login screen appears a pop-up warning message-box
also appears saying "your startup disk is full. You need to make
more space available on your startup disk by deleting files".
I click ok to remove the message and proceed with the usual login procedure.
I then enter the correct password, which should in turn re-direct me to the
Desktop as always. But, all that happens is I'm left stuck on the wallpaper
Background image of the login display screen rendering me incapable of accessing
any files for deletion in order to free up space on the drive.

I find this to be quite disturbing and frustrating.
If anyone can help me resolve this issue it would
be deeply and forever appreciated.

Thank you so much

problem solved!

Thank you so much for your reply.

Rebooting in Single-user mode didn't work though, but it led me in the right path to finding the solution.

* Rebooting in 'Safe' mode allowed me to regain access.

In case this happens to any other mac user...

1) shut down your computer
2) press the power button to turn the computer on
3) immediately after power activation, hold down the Shift key
4) let go of the shift key after you see that it's taking much longer to arrive at the Login screen.
5) When you arrive at Login area - At the top of the Login box it should say in red letters 'safe mode'
6) Log in as usual, and you're good to go!