VIDEO! Beyonce's Sister, Solange Assault Jay Z

Jay Z was seriously assaulted by Beyonce's sister Solange.. who can be seen in this VIDEO kicking and swinging at Jay Z inside an elevator from a surveillance video ... obtained by TMZ.

According to sources, this happened at a Met Gala after party last week at the Standard Hotel in NYC.  In the video ... Jay Z, Beyonce and Solange step into the elevator ... and then Solange goes crazy, screaming at Jay before unleashing a violent attack. 

Solange's attack was so fierce that a bodyguard attempted to hold Solange back on several occassion, but she manages to hit Jay Z at least 3 times.  At one point Solange throws a kick and Jay grabs her foot, but never attempts to strike her.  Beyonce stood by  inside the elevator without getting physically involved. 

It's unclear what triggered the fight.

You could see All three walked out of the building together -- Solange looks pissed -- but while Bey and Solange get in one car ... security walks Jay to a different one.