SHANGHAI — Fire engulfed a high-rise apartment building under renovation in China’s business center of Shanghai on Monday, killing 42 people and sending residents scrambling down scaffolding to escape, city authorities said.

The official Xinhua news agency cited a witness saying the fire began when building materials had caught alight. The blaze spread to scaffolding and then to the 28-story apartment block itself, which houses a number of retired teachers, it said.

The government said more than 100 fire trucks were called to battle the blaze and streams of water could be seen flowing into the building. The fire was largely put out about four hours later, it said. Firefighters could be seen taking bodies from the building.

Earlier reports put the toll at at least eight dead and more than 90 injured. The death toll had risen by Monday night to 42, the Shanghai government said in a statement, which did not give an updated figure for the number of injured.

Photos posted online showed a man clinging to the scaffolding. A resident identified as Mr. Zhou told Hong Kong broadcaster Phoenix TV that he and his wife were napping in their 23rd floor apartment when they smelled smoke. He said they climbed down the scaffolding four stories before being rescued by firefighters.

At one point helicopters could be seen hovering over the building, and witnesses said at least one resident was rescued, but thick smoke hampered further efforts. By evening the helicopters were gone.

The city government said that 100 people were rescued, but it was not clear if those included injured people.

A doctor at Shanghai’s Jing’an Central Hospital surnamed Zhang said more than 20 seriously hurt people had been admitted for treatment. Most of the survivors had suffered asphyxia from the smoke fumes, another doctor said.

Relatives searched hospitals for their loved ones. At Jing’an hospital, the father of Wang Yinxing, a 30-year-old woman who lived on the 22nd floor of the building, searched a list of survivors at the hospital but could not find his daughter’s name.

“She called her husband and said: ‘It’s on fire! I have escaped from the 22nd floor to the 24th floor,’ but then the phone got cut off,” the father, Wang Zhiliang, 65, said with tears in his eyes. “That was the last we heard from her.”

Shanghai state television showed survivors at another hospital in the city, covered in thick blankets as they emerged from a high-pressure oxygen chamber. Many survivors described climbing down the scaffolding to escape.

The state-run news website Eastday.com cited a construction worker surnamed Qian who escaped from the 28th story as saying crews were installing energy-saving insulation when the fire occurred.

Qian said thick, rolling smoke clouds surrounded the building and the room she was in filled with smoke, making it difficult to breathe, the report said. She said she called the city’s emergency hot line and then used a wet towel to cover her mouth and nose as she ran down a fire escape.

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Associated Press researcher Ji Chen in Shanghai and AP writer Gillian Wong in Beijing contributed to this report.