The incident occurred on the A21 Bromley Common between Crown Lane Spur and Pope Road, affecting traffic near Crown Roundabout.
Thames Water said its teams are “working round the clock” to repair the burst and restore the area back to normal.
To keep people safe, Bromley Common has been closed in both directions, with parts of the footpath also shut while emergency works continue.
Drivers travelling through the area have been warned to allow extra time for journeys as traffic diversions remain in place.
Several bus routes have also been affected, with the 61, 261, 320, and 358 currently on diversion due to the closure.
The road closure was later extended between The Crown and Hook Farm Road as repair work continued.
Traffic disruption has continued throughout the day, with the road still blocked in both directions while engineers deal with the burst main.
Thames Water apologised for the disruption and thanked residents and drivers for their patience while repairs are carried out.
The company said it would continue to provide updates on the situation, with the next update expected no later than 2pm tomorrow.
In a statement, Thames Water said: “We’re sorry to customers in the Bromley Common area who were affected by a burst water main overnight.
“We’ll be working round the clock to fix the issue and get the area back to normal.
“We apologise in advance for any disruption, which will be kept to a minimum.
“To keep everyone safe, we’ve had to close the road and parts of the footpath.
“If you’re travelling in the area please allow additional time to any journey as traffic diversions are in place.”
