Five people were killed when fire early Saturday engulfed their apartments above a row of stores in Brooklyn, the authorities said, as flames trapped residents on the second and third floors and caused interior walls to collapse.

A total of 125 firefighters were called to the three-alarm blaze, where they battled the heat and sudden structural damage as they tried to pull residents from inside two apartments above a

 sushi restaurant at 20-33 86th Street, in the Bensonhurst neighborhood.

Juan Gabriel, 18, who lived in the building next door, was awakened by another neighbor banging on the door. Mr. Gabriel said he and the neighbor ran out to the street and spotted Luisa Ordonez in the third-floor window of the burning building.

“She was screaming ‘help me, help me,’ ” Mr. Gabriel said. He said Ms. Ordonez was holding a 2-month-old baby boy, which moments later she tossed out the window to him and two other men below.

But in the darkness and the cold, the baby fell to the ground and sustained head injuries, the authorities said.

“She was screaming,” Mr. Gabriel said, “and I couldn’t do anything.”

Bars covered the lower half of the window — intended for child protection — but with fire and smoke building behind her, the bars prevented Ms. Ordonoz from escaping. She tried, but was unable, to climb out the window, Mr. Gabriel said.

Instead, she tossed a second child, a 2-year-old boy, out the window. He landed on the awning of a store below.

Firefighters initially found three bodies inside the building, and then in a further search, found two more.

“When you have multiple fatalities like this, it’s a very difficult day for all of us,” said John Sudnik, deputy assistant fire chief for the Fire Department of New York.

Three other people, including an adult, a 2-month-old and a 2-year-old — believed to be Ms. Ordonez’s children — were taken to Lutheran Hospital Center with injuries. Thirteen firefighters also sustained injuries, Mr. Sudnik said.

James Long, a Fire Department spokesman, said that one victim, a baby, had suffered a fractured skull, but the condition of the child was not immediately clear.

Mr. Gabriel and other people gathered at the scene said that most of the residents of the building were from Guatemala. The restaurant in the first floor was called H.K. Tea and Sushi.

Officials said the fire broke out at 2:31 a.m., and while several engine and ladder trucks quickly raced to the location, it was not until an hour later — at 3:30 a.m. — that firefighters found the first victims.

“It was delayed because of the volume of fire and the collapse,” Mr. Long said.

Firefighters brought the fire under control by 5:15 a.m., officials said.

At one point, as they battled from the interior of the “mixed-use” building, fire commanders on the scene urgently called for them to evacuate as part of the structure gave way.

“They pulled them out because the roof was about to collapse,” said a fire department spokesman. “They saw that the integrity of the floor of the ceiling was in jeopardy, so as a precaution they pulled everyone out.”

The injuries to the firefighters were not immediately clear.

Police and fire officials said the cause of the fire was under investigation. Fire marshals were searching through the rubble on Saturday morning as the fire commissioner, Salvatore J. Cassano, was headed to the location, Mr. Long said.