CAMBRIDGE, Mass.  Colleagues of Henry Louis Gates Jr., Harvard’s most prominent scholar of African-American history, are accusing the police here of racism after he was arrested at his home last week by an officer investigating a report of a robbery in progress.

Professor Gates, who has taught at Harvard for nearly two decades, arrived home on Thursday from a trip to China to find his front door jammed, said Charles J. Ogletree, a law professor at Harvard who is representing him.

He forced the door open with the help of his cab driver, Professor Ogletree said, and had been inside for a few minutes when Sgt. James Crowley of the Cambridge Police Department appeared at his door and asked him to step outside.

Professor Gates, 58, refused to do so, Professor Ogletree said. From that point, the account of the professor and the police began to differ.