Police and paramedics were called to the road's junction with North Castle Street at about 1pm on Tuesday.
Police were unable to rule out that his injuries may be life threatening.
Police confirmed that they are investigating the possibility the taxi driver suffered a 'medical incident' at the wheel in the moments before the collision.
Police also said that the 61-year-old male taxi driver and a female passenger were taken to hospital - but an ambulance service spokesman said this was precautionary.
Officers are still at the scene as inquiries continue, with North Castle Street to remain closed during the evening rush hour period.
In a statement released this afternoon, the force said: "Motorists are thanked for their cooperation and anyone who witnessed this, and has not yet spoken to officers, can contact Police Scotland via 101 and quote incident 1569 of 20 August."
Pictures sent to the Evening News show the front end of a black cab taxi which appears to have come to rest against the building - and the area has been cordoned off.
Another eye-witness said they saw three ambulances, more than one police car and two fire engines in attendance.