Police were called to High Street North, E12 at 8.30pm on Friday, 3 January with reports of a 70-year-old man in cardiac arrest.
London Ambulance Service also attended and the man was treated at the scene and taken to hospital in a critical condition.
Prior to the man being found collapsed in the street, it is believed he was chasing a suspect possibly thought to have attempted a burglary at a residential property in Plashet Grove, E6.
The man remains in hospital where we await a further update on his condition.
No arrests at this time.
Enquiries ongoing to establish the full circumstances of the incident