The air ambulance landed in Peterborough at lunchtime today to attend an emergency medical incident.
Police, two crews from the East of England Ambulance Service and the East Anglian Air Ambulance were called at 11.59am to Windmill Street at the junction with Lincoln Road.
A man in his 70s suffered a cardiac arrest.
The patient was treated at the scene and then taken to Peterborough City Hospital in critical condition.
Motorists are advised to avoid the area.
Stagecoach tweeted: "Service 1 delays in both directions due to an air ambulance & other emergency vehicles attending scene of incident. Please be patient, thank you."