The grass fire in Norbury, South London covers around six hectares, equivalent to 60,000 square metres.
School fields as well as wasteland and shrubbery have caught alight.
Locals are being warned to keep windows and doors shut to protect against the massive plumes of smoke emerging from the blaze.
There are even reports on social media that people have been forced to evacuate their homes in the area.
Fifteen fire engines and around 100 firefighters are on the scene near Turle Road.
The London Fire Brigade have warned that the fire “is producing heavy smoke”.
Pictures show the smoke as visible from miles away.
One social media user said they were “praying they get it under control”.
Another on the scene said: “There’s a massive fire in Streatham.
“I’ve never seen so many emergency vehicles, the common is covered in eye burning smoke, the column of smoke is f***ing enormous.
“Something really bad has happened.”
Station Commander Keith Sanders, who is at the scene, said: “Firefighters are working hard to bring the fire under control.
“The fire is producing smoke in the local area and the direction of the wind is blowing smoke towards local terraced houses.
“Please avoid the area and if you live nearby, keep your windows and doors closed.”
User @DawnSmith1970 added: “Ash falling through my skylights, smoke blocking out the sun. Quite freaky.
“Lots of sirens. Hope everyone is OK.”
The Brigade’s 999 Control Officers have taken over 70 calls to the blaze.
London Fire Brigade has said that the cause of the fire is not known at this time.
It was first called at 12.27pm to a grass fire near Turle Road in Norbury.
Fire crews from West Norwood, Mitcham, Wallington, Norbury, Woodside and surrounding fire stations are currently tackling the blaze.