Homicide detectives are investigating after a 47-year-old Melbourne man died in the back of an ambulance.

The man had been found at an intersection in Gardenvale, in the city's south, on Wednesday evening after police received a report that he wanted to harm himself.

An officer escorted the man into an ambulance.

He was then treated by paramedics but died while still en-route to hospital, a police spokeswoman said.

The homicide squad and the professional standards unit have been called in since the man died in the presence of a police officer.

A cause of death is not known.

* Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14 or Suicide Call Back Service on 1300 659 467