Officers were called to St Swithun's in the south east of the capital after a stabbing on Saturday night.
It's understood a person who was not invited to the 16th birthday party stabbed a 23-year-old man before a fight spilled onto the street.
Distressed locals took to social media to describe the scene as "like a war zone" and a "riot".
Others said there were huge numbers of police with dogs as the youths continued to fight.
The church is not thought to have been linked to the incident.
A Metropolitan Police spokeswoman said: "Police were called to St Swithuns Road, SE13 at around 2300hrs on Saturday, 30 May to reports of a person having been stabbed at a 16th birthday party.
"On arrival officers found a 23-year-old man suffering from a stab wound to his side. He was taken to a south London hospital by London Ambulance Service. His injuries are not life threatening.
"A group of around 50 people loitered in the area and were the cause of some disorder.
"Five people were arrested for offences relating to public order. They have been taken to south London police stations.
"Officers from Lewisham supported by dog units and Air Support dispersed the group. Enquiries continue."