It is believed a number of people have been stabbed in the incident at Streatham High Road, police said.
Witnesses on social media have said they heard three gun shots fired just after 14:00 GMT on Sunday and people were being treated by doctors.
London Ambulance Service said it had a number of resources attending the incident.
One person tweeted: “Something major happening in Streatham High Road. Armed police and roads closed off”.