Usher Performs Vegas Show Hours After Lovers & Friends Fest Canceled

Usher said the show must go on ... pulling up to perform a show in Vegas despite the Lovers & Friends festival's last-minute cancelation -- and raising fans' spirits.

Video obtained by TMZ shows the popstar taking the stage at Jewel Nightclub Aria Hotel and Casino Friday night ... and, belting out some tunes with the audience after news of the festival's cancelation came in.

Check out the clip ... Usher arrives in an all-white get-up, drink glass in hand -- and, he immediately gets the crowd involved, making them sing the opening notes to a track and hyping up the whole joint.

Lil Jon also pulls up ... and, together they perform their hit song "Lovers and Friends" for the crowd -- a meaningful choice given the bad news the audience received hours before Usher pulled up.

Our eyewitnesses say Usher arrived at 1:30 AM Saturday to play for the club ... about three hours after festival organizers posted news of the concert's cancelation due to severe weather.

The pre-show where he performed was already apparently planned, but, it appears some thought he may cancel after the festival itself got the axe. Instead, Usher showed up and showed out ... and, we're told the party went on until 4 AM.

BTW ... there's video floating around on social media showing what some online are claiming is a disappointed Usher sinking to his knees during his soundcheck -- this allegedly occurring just after organizers sent out the cancelation message.

Admittedly, the vid's pretty blurry, and it's hard to tell much of what's going on ... but, the figure is wearing all white like Usher did to the club -- so, it's possible it's him. Hard to say.