Station commander Gary Deacon said from the scene: “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.”
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 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.”
Ms Dolcy-Grant lives on the second floor of her building so her home was not flooded, but at noon on Saturday she said she still had no running water, all shops were closed and transport had been halted on her street.
Thames Water said local houses in north-west London, in the postcodes of NW2, NW6, NW8 and NW10, could be experiencing no or low water pressure as a result of the burst main.
The company said in a statement: “We’re sorry to customers in NW6 and the wider area who have no water or lower pressure than normal today due to a burst pipe on Belsize Road.
“Loss adjusters are in the area to assess the damage and support residents in getting their lives back to normal once the water has receded.”