The man, who was not immediately identified, was in custody and no one was injured, police spokeswoman Sgt. Kimberly Schneider said.
An officer first spotted the man near Union Station, about two blocks from the Capitol.
Police used a robotic camera to see inside the vehicle but could not determine some "items of concern," Schneider said.
Authorities were evacuating parts of a nearby building as a precaution so they could use a powerful water hose to destroy the vehicle.