Rihanna capped off a night of partying in London with a bloodied knee after someone allegedly threw a bottle in her direction ... and screamed things at her about Chris Brown.

According to reports, RiRi was leaving a club called The Box when someone threw a bottle of Lucozade (a UK energy drink) in her direction. The bottle didn't connect, but Rihanna allegedly scraped her knee during the commotion when she brushed up against a metal shop grate.

Rihanna was in town for London Fashion Week.

We reached out to a rep for RiRi -- so far, no word back.