The 23-year-old had just arrived in the British capital before the final three shows of her Loud tour and decided to put on a prom party.
The We Found Love singer is said to have gone all out with her attire.
A source told British newspaper The Sun: 'Rihanna didn't scrimp on detail. She walked around the hall dressed like Madonna and put wrist corsages on all the girls.
'When she helped serve the dinner, she was teasing the guests with the zip on her dress.'
Rihanna has two dates left on her Loud tour which finishes tomorrow at London's O2 Arena after a performance at the same venue tonight.