Russell Brand In Custody -- Citizen's Arrest

Russell Brand has just been arrested for allegedly attacking paparazzi at LAX airport ... TMZ has learned, and it was a citizen's arrest.

Russell Brand arrest video.
As we first reported, Brand and Katy Perry were going through Delta security when he allegedly began pushing and striking paparazzi.

We're told a photog -- one of the alleged victims -- made the citizen's arrest and LAX cops facilitated it.

Cops interviewed Brand, then took him into custody. Cops say he's being booked for battery.

Check out this video of another angle of the incident.

UPDATE 5:54 PM PT: Russell has posted $20,000 bail and has left the police station.

Around the same time, Katy defended Russell via Twitter, saying, "If you cross the line & try an put a lens up my dress, my fiancé will do his job & protect me."

The alleged upskirting is not seen in video of the incident.