As Washington braces for a “white civil rights” rally and counter-protests, the police are investigating an attack in which the victim says a cyclist hurled racial slurs and hit him with a bike lock.

The victim, Ketchazo Paho, 34, of Maryland, said he was driving through Georgetown on his way home early Monday when he was assaulted by the bicyclist who struck him in the head with the metal lock.

Mr. Paho said he fears similar attacks could happen this weekend amid plans for a “Unite the Right” rally and counter-protests this weekend.

The Metropolitan Police Department arrested a white suspect, Maxim Smith, and charged him with one count of assault with a dangerous weapon. The police said they were investigating to determine if the crime was motivated by hate or bias.