The victim, in his 40s, had been in a pub with his partner in Central Road when a group of men approached and invited them to sit outside.
Despite being friendly initially, the group started to make homophobic comments and the couple moved away.
After leaving Centrals Bar and Lounge alone, the victim was set upon by a man as he walked Central Road between 1-2am.
The victim recalls homophobic comments being made before he was punched hard in the face.
Acting Detective Sergeant Herdeep Jugdev, from Bromley CID, said: “This was an extremely violent attack which left the victim with head and facial injuries for which he is still receiving hospital treatment.
“It also appears that the assault was motivated by the victim’s sexuality. Hate crime of whatever kind is something we will never tolerate and we are doing all we can to find the person or people responsible.
“We would ask anyone who was on Central Road on Saturday evening, particularly anyone in Centrals Bar and who may have seen any interaction between the victim and other people, to come forward and speak to us.”
Police are also appealing for anyone driving along the road between 1.10am and 2am who may have dashcam footage to check it.
Anyone with information should call 101 with CAD reference 2214/20JUN. Alternatively contact Crimestoppers 100 per cent anonymously on 0800 555 111 or online at crimestoppers-uk.org.