Birmingham during the morning rush hour.
Police said officers responded to a call from a member of the public reporting that a passenger had been attacked on Hagley Road, in the city centre, shortly after 7.30am on Thursday.
A spokesman for West Midlands police said he could not confirm reports as to the age and gender of the victim. He said: "Officers and other emergency services were immediately dispatched but it was clear that nothing could be done to save the victim. West Midlands police have identified the victim's family and are in the process of breaking the sad news."
A National Express West Midlands travel spokesman said: "We can confirm that a tragic incident took place on a No 9 service on the Hagley Road this morning.
"Our thoughts and condolences are with the family of the person involved and we will offer all possible support to the police in their investigation into the matter."
The Hagley Road, out of city, was closed after the incident and police said it was unlikely to reopen on Thursday.
West Midlands ambulance service said it had sent an emergency doctor, a senior paramedic and two ambulance crews to the scene of the incident. A spokesman said: "One person was confirmed dead at the scene."