Police are appealing for witnesses to a collision between a pedestrian and a cyclist taking part in the Ride London event on Sunday, 30 July.
Officers were called just after 12:00hrs midday to the incident, which took place at the junction of New King's Road and Guion Road in the Parson's Green area of Hammersmith and Fulham. London Ambulance Service also attended.
A male cyclist in his 60s was taken to hospital where he was treated for minor injuries, and subsequently discharged. The pedestrian, a woman aged 67, suffered head injuries and was also taken to hospital. She remains there in a critical but stable condition.
DC Amanda Hawke, from the Serious Collision Investigation Unit, said: "We really need to understand more about what happened and hear from anyone who saw this collision, whether they were members of the public or perhaps working as part of the event. Please call into the incident room if you have any information."
Enquiries continue into the circumstances of the incident. Anyone with information can call the Serious Collision Investigation Unit number on 0208 543 5157, or via the police 101 non-emergency number.