Four fire engines and around 25 firefighters attended a fire on Maple Road in Penge.
Part of a ground-floor takeaway was damaged by fire. Part of a flat above the takeaway was also damaged. Two people were taken to hospital by London Ambulance Service with a further two people assessed at the scene.
The fire is believed to have been caused by a hot grill that had been left unattended and then ignited ducting.
A London Fire Brigade spokesperson said: "Cooking should never be left unattended on the hob or grill - if you have to leave the kitchen, turn off the heat.
"We'd like to remind restaurant and takeaway owners to ensure they don't have dirty ducting. If you don't clean your extraction system regularly, you're at a greater risk of ducting fires.
"You should also make sure you keep the hob, cooker hood and extractor fan clean too – built up fat and grease can ignite and cause a fire."
The Brigade received the first of four calls at 1002 and Control Officers mobilised crews from Beckenham, Woodside, Forest Hill and West Norwood fire stations to the scene. The fire was extinguished by 1307.