The fire service responded to flooding covering "around 800 metres" of Camden in the early hours of Saturday morning (December 17), following a burst water main.
Neighbours in Belsize Road reported residents of basement flats being rescued as waters rose above half a metre.
London Fire Brigade said around 100 homes were affected by flooding, while "thousands" were left without water.
Thames Water said households were left with "no water or low pressure" in NW2, NW6, NW8 and NW10.
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One resident said: "The fire brigade have been amazing - rescue-boated a pregnant lady
"I couldn’t tell if the pregnant lady actually got into the boat or not in the end, but they were excellent in getting to her aid
"No casualties - the ones opposite were asleep in the basement flats but woke up whilst it was filling, thankfully."
London Fire Brigade (LFB) said eight fire engines and around 60 firefighters responded to the call to Belsize Road in South Hampstead at about 2.50am.
(Video: George)
Stefano Calcagni, from Hammersmith, was dogsitting for a friend in a first and second-floor property in Belsize Road when he woke to a text telling him about the floods.
The 48-year-old banker was evacuated by boat alongside the two hounds, named Batista and Pandora, on Saturday afternoon, after watching his neighbours in the basement flat below escape via their window.
“I opened the curtains, I saw there was water downstairs,” Mr Calcagni, originally from Milan, Italy, told the PA news agency.
“The water level was halfway up the door to the basement flat.
“It’s owned by a couple, I saw them leave through the window as they couldn’t get out the door.
“I’ve spoken to my friend and told him about everything, he’s back tomorrow but knows it’s not as bad as for the basement flat – one week before Christmas.”
Mr Calcagni said water had come up the outside steps to his friend’s home and not inside, but he decided to evacuate to stay with another friend as there was no central heating, no hot water and little food in the house.
“[Being on the boat] was surreal, there were things floating everywhere… the dogs got excited on the little boat but they’re fine,” Mr Calcagni added.
A 42-inch water main had burst, causing flooding "to a depth of around 50cm across an area of around 800 metres", according to LFB.
Station commander Gary Deacon said: “This is a significant flooding that has affected around 100 properties and left thousands without water.
“Shift 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 have decided to remain in their properties and we’re patrolling the affected area to assist any further people who require assistance.
“The water authority are working to isolate the burst water main and firefighters are expected to remain at the scene throughout the afternoon.
“A rest centre has been set up for affected residents at Swiss Cottage Leisure Centre.”
(Video: George)
Thames Water said in a statement at just before 2pm on Saturday: "Our engineers have now shut off the flow from a large burst pipe in Loudoun Road.
"It’s taken time to achieve this safely, as they needed to switch off a number of valves at different locations, some of which were under parked cars or submerged by flood water.
"They also needed to make sure when operating them that this didn’t lead to a further burst pipe elsewhere.
We have loss adjusters and support staff at Swiss Cottage Library, where Camden Council is running a rest centre.
"They’ll be talking to residents who’ve been flooded, to help get their lives back on track.
"We’re really sorry for the flooding and distress this has caused, as well as the impact on local water supplies.
"Once we’ve been able to turn off the water, we’ll focus on refilling the network – we don’t yet know how long this will take but will update you as soon as we can. In the meantime, customers who are without water should still be able to use their heating."
(Video: George)
Camden Council said its emergency team was working with LFB and Thames Water.
The council said in a statement: "The advice from the London Fire Brigade at present is for residents directly affected by the incident to stay in their homes on the upper levels.
"A rest centre has been established at Swiss Cottage Leisure Centre and residents can attend there once the instruction from the LFB changes and residents can leave their homes.
"Thames Water continue to work to fix the main and ambulance and support services are on site to attend to any emergency health needs.
"The London Fire Brigade are advising the public to stay away from the area."
Cody Dolcy-Grant lives in Hilgrove Road, at the end of Belsize Road, and said she woke up to scenes “like the river Thames” outside her home.
“I wasn’t aware of the situation ’til 7am when I went to the toilet, realised the toilet wasn’t flushing and no water was coming out of the taps,” the 27-year-old, who is unemployed due to a health condition, told the PA news agency.
“My road was flooded… when I opened the front door it was like a horror scene, fire engines up my whole road, police, ambulance, and then I looked to my left and it was like the river Thames outside.”
Brent councillor Ketan Sheth told this newspaper: "The burst itself occurred on Loudoun Road so the water company is now using valves to isolate this section of pipe. This is taking time because they need to carry this out in a safe way, so they do not cause further bursts on the network due to pressure changes."
Fire crews from West Hampstead, Kentish Town, Euston, North Kensington and surrounding fire stations attended the scene.
Residents who need help after being affected by the incident should call Thames Water on 0800 316 9800.
More to follow.
Email andre.langlois@newsquest.co.uk
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