Officers were first called to an incident in Chapman Road, Croydon at around 6.55pm on Friday.
A male victim with non-life threatening injuries was found.
At 7.15pm, police were alerted by a south London hospital, after four males, all in their 20s, were admitted to casualty with slash and stab injuries.
One of the males is currently believed to be in a serious condition.
Police then responded to another call from a south London hospital at 8.51pm, after another two males were taken in suffering from knife wounds.
One of the victims is said to be fighting for his life.
Officers then arrested a man on suspicion of stabbing, after being called out to Dingwall Road, Croydon at 9.12pm