Met Police are investigating the incident in Streatham, where a number of people are believed to have been stabbed.
In a tweet, the The Met Police said: " A man has been shot by armed officers in Streatham.
"At this stage it is believed a number of people have been stabbed.
"The circumstances are being assessed; the incident has been declared as terrorist-related."
The force advised people to avoid the area while emergency services deal with the incident.