WWW- If you invite Motley Crue singer Vince Neil to perform at your charity fundraiser, better get him onstage before his inner demons take over.

I hear that Neil, 44, was fine during the afternoon rehearsal and sound checks before Saturday’s black-tie “Puttin’ on the Ritz” gala in Miami, which raised $2 million for a school that rehabilitates juvenile offenders.

But some time between then and showtime – around 10:30 p.m. – the wheels came off.

According to multiple witnesses, the rocker swaggered through the high-powered crowd of South Beach denizens, judges and lawyers – including star defense attorney Roy Black, whose wife, Lea, chaired the event – brandishing a microphone and spewing profanities.

“F- you!” he shouted at another entertainer, drag queen Elaine Lancaster. “I bet you’d like to!” Lancaster retorted.

Then Neil attempted to lead his stunned audience in a spirited chant of f-words. “I was disgusted,” said a Lowdown spy.

Finally, Lea Black intervened, gripping him tightly to steer him onstage, and persuaded him to sing. “I was just so grateful that he agreed to do this,” Black told me yesterday. “He’s a big-hearted guy who meant well.”