A 26-year-old man suffered serious head injuries during the fight in Holborn Way, Mitcham, on Monday, and died two days later.
The victim has not yet been formally identified. Two other people were also hurt in the the brawl.
A seventh suspect, a 25-year-old man, was arrested on Friday on suspicion of murder and remains in police custody.
Previously, police arrested a 21-year-old man on suspicion of attempted murder. Five other men, aged between 21 and 40, were held on suspicion of violent disorder and all six have since been bailed.
Scotland Yard said the fight at about 19:15 GMT on Monday involved "a large number" of men.
A seventh man has been arrested as part of an investigation after a fatal street brawl in south London.