Two teenage men were hurt when they tried helping another youth who was being attacked by up to 10 people outside a pub in Erith, police say.
The fight happened shortly after midnight on Sunday outside The Ship in West Street.
It is thought an 18-year-old man had an exchange with another man which then led to him being attacked by a group of people.
The teenager was found collapsed by police and taken to hospital suffering a head injury.
Two other 18-year-olds believed to have been trying to help the victim were also injured, one suffering a head injury that needed hospital treatment and another who had bruising to his face.
Police said the injuries of the two youths taken to hospital were not life-threatening.
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Suspects are described as a group of men and women in their mid to late teens who ran off towards St Fidelis Road.
Bexley police are appealing for witnesses.
Anyone with information is asked to call Bexley CID on 101 or Crimestoppers anonymously on 0800 555 111.