Eight fire engines and around 60 firefighters were called to flooding on Belsize Road in Camden.
A 42-inch and a 15-inch water mains burst, causing flooding to a depth of around 50cm across an area of around 800 metres. Around 100 properties were affected by flood water.
Firefighters and Hazardous Area Response Teams led around 25 people to safety.
The Brigade's 999 Control Officers took 18 calls to the flood.
Station Commander Gary Deacon, who was at the scene, said: "This was a significant flooding that affected around 100 properties and left thousands without water. Swift water rescue crews used boats to move around 20 people to upper floors of properties. Three adults and a child were evacuated to a place of safe haven.
“A number of residents decided to remain in their properties and firefighters patrolled the affected area to assist any further people who require assistance.
"There were numerous road closures in place and we asked people to avoid the area where possible.
"Crews used flood barriers and a high volume pumping unit to divert flood water.
"A rest centre was set up for affected residents at Swiss Cottage Leisure Centre."
The Brigade was called at 0250 and the incident was over for firefighters by 1938. Crews from West Hampstead, Kentish Town, Euston, North Kensington and surrounding fire stations are at the scene.