A man trying to cross Route 9 in Brookline was slapped in the face by a driver who stopped on part of the crosswalk, police said.
Brookline police said that on Wednesday the victim reported the incident to officers.
The man said he was trying to cross Route 9, also known as Boylston Street, at Tully Street on Wednesday morning when a car pulled across the crosswalk.
The man then motioned for the driver to move, which apparently enraged the driver.
Police said the driver responded by "exiting his vehicle and slapping the victim on the left side of his face, knocking him to the ground."
After the assault, the driver got back in his car and took off, police said.
The suspect is described as a white man in his late 50s with a gray mustache and a stocky build.
Follow Michael Naughton on Twitter @metrobosmike.