Six people were killed in a massive fire which engulfed Grenfell Tower, a 24-story apartment block in West London, Metropolitan Police said.
The number of fatalities is expected to rise.
74 people were taken to hospitals across the city on Wednesday, with 20 in critical conditions, according to the city's Ambulance Service.
London Fire Commissioner Dany Cotton said a number of firefighters were also injured, although not seriously.
"This is an unprecedented incident," she said. "In my 29 years of being a firefighter, I have never ever seen anything of this scale."
The fire, which reportedly started at 1:16 a.m. local time (0016GMT), began on the second floor of the 24-story building, according to the London Fire Brigade.
Ambulances stationed near Grenfell Tower. /VCG Photo
A witness told the BBC that the blaze escalated and quickly spread to the upper floors. People were screaming for help and throwing children out to safety.
A baby was dropped from an apartment on around the 10th floor and caught by a man.
At least 40 fire engines and 200 firefighters were dispatched to tackle the blaze. Police have cordoned off the area around the tower block and evacuated around 30 adjacent flats.
The fire reportedly started on the second floor and quickly spread to upper levels. /VCG Photo
"The police keep pushing back their cordons, pushing back members of the public for fear the building might collapse," the BBC reported, citing a correspondent at the scene.
A spokesman said Prime Minister Theresa May was "deeply saddened by the tragic loss of life in Grenfell Tower."
Mayor of London, Sadiq Khan, said on Twitter that "major incident" status had been declared. He advised survivors to contact emergency services so that they won't be counted as casualties.
Grenfell Tower, built in 1974, contains 120 homes, according to Kensington and Chelsea Council. The Local Grenfell Action Group said the building had been a fire risk and access to the site may have been "severely restricted," according to the BBC.
A man living on the 17th floor told local media that there was no fire alarm in the building and that residents had long been concerned about the building's safety.
"Something has gone dramatically wrong here," said Geoff Wilkinson, a fire and building inspector. "If there is a fire in any of these buildings, you'd expect it to be contained to an individual apartment. You wouldn't expect it to spread in anything like the way, and certainly not in the time, that we've actually seen here."
Former Mayor of London Ken Livingstone has called for tower blocks to be "re-examined" to check they are "fit for purpose."
An investigation into the cause of the fire is still underway.
Fire rips through Grenfell Tower as firefighters attempt to control the blaze./VCG Photo
How to escape a fire in a high-rise building
· Heed the alarm. If the fire alarm sounds, do not assume it's a test or a false alarm and do not wait to see what happens.
· Protect yourself from smoke. Smoke is toxic and many caught in fires die from smoke inhalation.
· Never use the elevator. Always take the stairs to exit as elevators may malfunction during a fire.
