The 24-year-old appeared as a mermaid before performing at the event.
She was joined by Pamela Anderson and Desperate Housewives star Eva Longoria at the Life Ball.
Former Baywatch actress Anderson, 41, performed a bizarre routine with a ghetto blaster but wowed everyone with her sparkly gold leotard.
Thousands of revellers in quirky &ndash and often kinky &ndash costumes joined in the annual bash at Vienna's city hall on Saturday.
The spectacular opening ceremony was peppered with pleas from big stars &ndash including former US president Bill Clinton &ndash not to give up the fight against HIV and Aids.
Glam rock: Pamela Anderson turns on the crowd with her ghetto blaster
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When the spotlight gets too much for celebrities they like to don a disguise and in some cases, not human ones. here