Britney Spears has truly lost it!  Now she's fighting back at the paparazzi.

Just hours before heading back to rehab for a third time on Thursday, Brit-Brit evidently wanted to say good-bye to the babies who just happened to have been staying with her estranged hubby, Kevin Federline.

When Brit arrived at K-Fed's place, she rang the bell three times to no avail.  Either daddy and the kids went out for some vodka and cigarettes or they were inside hiding from the "Toxic" singer. 

Spears got so mad that she attacked the paparazzi.  X-17online caught it all on camera.  She picked up a patio umbrella and beat it against one of the photographer's cars, not to mention the psycho look in her eyes while doing it.

Maybe she'll stay safe in rehab while making an attempt to rebuild herself.