Brown faces felony assault charges after allegedly attacking the man, TMZ reported.
Sources told TMZ Brown didn't appear to be under the influence of drugs or alcohol, and he and the alleged victim didn't appear to know one another. The source said the alleged victim never struck Brown.
The man Brown allegedly punched was being treated at a Washington hospital.
Police also arrested Brown's bodyguard, ABC News reported.
Brown was being held for court Monday because he is currently on probation and not eligible for release until he sees a judge, a source told ABC News.
Brown is on five years probation after pleading guilty in 2009 to a felony for assaulting then-girlfriend, singer Rihanna.