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Rihanna took for for London from LAX Sunday afternoon in a Donna Karan one-piece sweatsuit with heels ... an interesting combo!

Ri was flanked by bodyguards the night before as she hit up Playhouse with no fewer than FIVE bodyguards in tow!

The singer beefed up security after a crazed man threw a bottle at her in London a few weeks ago. The incident didn't stop Rihanna from her hard-partying ways, however ... she left Playhouse nightclub in LA Saturday night with her "friend" Chris Brown at 3:30am. Innocent enough; right???