The pipe, which carries drinking water, burst at Mitcham Lane in Tooting at around 11:00 BST.
Local traffic in the area was disrupted by the burst. Thames Water apologised to anyone affected.
A spokesperson said: "Our engineers are there and planning how best to stop the flow of water with minimal disruption to our customers' supply.
"We'll do all we can to get things back to normal as quickly as possible."
Sian Rowland tweeted: "Impromptu water feature in Mitcham Lane. Avoid the area it's chaos!"
Wandsworth Police tweeted:  "Road closure currently affecting SW16 - Mitcham Lane closed at junction with Southcroft Road due to severe burst water main."
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