Thick black smoke filled the air in Brixton after a fire broke out at a 16-storey block of flats.
More than 70 firefighters and 10 fire engines were called to Canterbury Crescent at 1.19pm on Saturday after a blaze broke out at a ground floor property.
The building was evacuated, with more than 50 people forced to flee their homes.
A pregnant woman, girl and a man were treated by paramedics for smoke inhalation before being taken to hospital.
The flat where the fire began was badly damaged and flats on the first and second floors were also damaged in the blaze, London Fire Brigade said.
Witnesses said the blaze was sparked by an "explosion" at the block.
One witness told the Standard: "We were having lunch and heard a small explosion and we saw a black cloud of smoke going up and the growing fire.
"Some seconds after the first explosion he heard one or two more, also small. There were some people in the field in front, I guess calling the emergencies."
Pictures of plumes of smoke billowing from a block of flats close to Brixton station have been posted on social media, with the smoke clearly visible from the railway line.
The fire was brought under control by 2.44pm.
The cause of the fire is under investigation.