LONDON — They banged on windows, screamed for help, dropped children from smoky floors in a desperate attempt to save them. Terrified residents of the Grenfell Tower said there was little warning of the inferno that engulfed their high-rise apartment building and left 12 people dead — a toll that officials said would almost certainly rise.

The blaze early Wednesday in the 24-story building in west London’s North Kensington district also injured 74 others, 18 of them critically, and left an unknown number missing. A tenants’ group had complained for years about the risk of a fire.

More than 200 firefighters worked through the night and were still finding pockets of fire inside later in the day. A huge plume of smoke wafted across the London skyline and left a burned-out hulk in the working class, multi-ethnic neighborhood.

“In my 29 years of being a firefighter, I have never, ever seen anything of this scale,” Fire Commissioner Dany Cotton said.

Up to 600 people lived in 120 apartments in the Grenfell Tower. After announcing the updated death toll of 12 in the afternoon, Cmdr. Stuart Cundy said that “we believe this number will sadly increase.”

Crews rescued 65 people, said Steve Apter, the fire brigade’s director of safety and assurance.

Prime Minister Theresa May’s office said she was “deeply saddened by the tragic loss of life” in the fire.

“My thoughts are with the victims, their families and all of those who had their homes destroyed,” she said. “It’s impossible to comprehend the horror of what they’ve been going through.”

London Mayor Sadiq Khan said many questions must be answered about safety for the scores of other apartment blocks around the British capital.

The London Fire Brigade said it received the first reports of the blaze at 12:54 a.m. and the first engines arrived within six minutes.

Survivors told of frantic attempts to escape during the nighttime fire. Some initially feared it was terrorism-related, although authorities have not suggested that terrorism was involved.

“The flames, I have never seen anything like it. It just reminded me of 9/11,” said Muna Ali, 45. “The fire started on the upper floors. … Oh my goodness, it spread so quickly. It had completely spread within half an hour.”

Samira Lamrani said she saw a woman drop a baby from a window on the ninth or 10th floor to people on the sidewalk.

“People were starting to appear at the windows, frantically banging and screaming,” Lamrani told Britain’s Press Association news agency.

When the woman indicated she was going to drop the infant, “a gentleman ran forward and managed to grab the baby,” she added.

Joe Walsh, 58, said he saw someone toss two children out a window on the fifth or sixth floor. Tiago Etienne, 17, said he saw about three children between the ages of 4 and 8 being dropped from around the 15th floor. There was no word on their fate.

Other residents told harrowing tales of their own escapes and frustration at not being able to help neighbors.

Ruks Mamudu, 69, said she ran to safety down one flight of stairs to the ground floor from her apartment wearing only her purple pajamas and bathrobe. She and her grandson sat outside the building, helplessly watching those trapped on higher floors.