Backstreet Boys Surprise Fans at Wild Las Vegas After-Party, on Video

The Backstreet Boys are ready to “Get Down” in Las Vegas all weekend … pulling up to a club after their show at the Sphere in Sin City Saturday night.

We’ve obtained video from the Voltaire Belle de Nuit … a popular club at the The Venetian on The Strip — where all five members of the band pulled up around 12:30 AM last night.

Check out the clip … AJ McLean makes a big impression with the ladies in the crowd who are screaming his name and hugging him as he wades through them in the club.

He doesn’t seem to mind the furor … and, even laughs it off when someone grabs him from behind — though the people around her caution her to be respectful.

McLean then jumps onstage to rock out to “Hypnotize” by the Notorious B.I.G. … ripping off his shirt to bounce around onstage to the track.

AJ certainly stood out the most from the group … along with Kevin Richardson who sang the Fugees hit “Killing Me Softly with His Song.”

While Nick Carter, Brian Littrell and Howie Dorough only hung out for about 30 minutes we’re told, AJ and Kevin were there for an hour before taking off … a pretty long time for five dudes who just rocked a packed house at the Sphere.

If you don’t know … the Backstreet Boys just kicked off a residency in Las Vegas in the high-tech Sphere — and, “Everybody” is going crazy for the ’90s boy band.