Officers are appealing for anyone who may have witnessed a serious assault onboard a bus in New Cross Gate, to come forward with information.
Police and the London Ambulance Service (LAS) were called at approximately 00:20hrs on Tuesday, 13 March to reports of an assault on the number 53 bus in New Cross Road, near the junction with Alpha Road.
The victim, a 36-year-old man, was approached by two male suspects on the lower deck of the bus.
Suspect 1 is believed to be a light-skinned black man, aged in his 20s, and was wearing beige coloured trousers.
Suspect 2 is described as a black male wearing a dark coloured hoodie.
The victim suffered a severe head injury and was taken to a south London hospital, where he remains in a critical condition.
Detective Chief Inspector Tony Lynes, from the Met's Specialist Crime and Operations Command, said: "We are appealing for any witnesses who may have been a passenger on the bus at the time the assault occurred and potentially anyone that may have recorded events on mobile phones.
"We're also appealing for anyone else who may have any information regarding the suspects involved who, following the attack made off towards Amersham Vale."
Anyone with information can call the enquiry team on 020 8721 4868. Alternatively information can be given anonymously by calling Crimestoppers on 0800 555 111.