The blaze tore through the semi-detached house shortly before 1am on Saturday, completely gutting the building.
Six fire engines and 30 firefighters attempted to tackle the blaze, but a 41-year-old woman, a nine-year-old girl, two boys aged five and two, and two teenage girls aged 14 and 13, were killed in the fire.
Two others, a 51-year-old man and a 16-year-old girl, were taken to hospital with burn injuries after escaping from the building as flames engulfed the ground and first floor. The girl was in a critical condition while the man was stable, police said.
It is believed that all the victims are members of the same family.
The fire is being treated as unexplained, and the Metropolitan Police and LFB have begun an investigation, a police spokesman said.
Glen Gorman, London Fire Brigade station manager, said: "There's no doubt this is an absolute tragedy.
"My thoughts, and those of my colleagues at the London Fire Brigade, are with the friends, family and loved ones of those affected at this extremely sad and difficult time.
"We are now working with the police to ensure a full and thorough investigation is carried out."
A spokesman for the fire brigade said the blaze was under control by 2.50am on Saturday. The ground floor and first floor of the two storey house have been badly damaged and the windows have been blown out by the intense fire.
Post-mortem examinations were due to take place later.
Firefighters from Willesden, West Hampstead, North Kensington and Park Royal fire stations were involved in tackling the blaze. They were joined by officers from the Met and medics from London Ambulance Service.
A LFB spokesman said: "It is understood that there were six fatalities - it is thought that this may be three children, two teenagers and one adult, but we await confirmation.
"Two further people escaped from the house before firefighters arrived at the scene, both were injured and were taken to hospital by ambulance."
An LAS spokeswoman said the two injured victims, a 55-year-old man and 16-year-old girl, were taken to Saint Mary's Hospital in Paddington.
She told the BBC: "We sent the air ambulance doctor in a car, four ambulance crews and a hazardous area response team. Sadly four youngsters and an adult were dead at the scene.
"Our staff treated a five-year-old boy in cardiac arrest but despite extensive efforts they were unable to resuscitate him."
A police spokesman added: "We are working with the fire brigade to establish the circumstances of the fire but it is currently being treated as unexplained."