SHANGHAI, China — Chinese state media reported that unlicensed welders accidentally started a fire in a high-rise apartment building in Shanghai that has so far left 53 dead.

Police detained the four welders on criminal charges Tuesday, a day after the fire broke out in the 28-story high rise that mainly housed retired teachers.

In addition to the dead, more than 70 people remained hospitalized, and family members were still searching hospitals for unaccounted relatives.

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, which was largely put out about four hours later. Firefighters could be seen taking bodies from the building, while survivors were rushed away in ambulances.

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Other survivors were housed overnight in a gym of a nearby retirement home.

There were sad scenes at hospitals as relatives searched 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."

Some residents escaped by climbing down scaffolding that had been put up for the renovations. 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.

An unidentified woman told Shanghai television her only option was to climb down the scaffolding. "If I jumped I would die, if I stayed (in the building) I would die," she said.

Public Security Minister Meng Jianzhu, China's top policeman, called Tuesday for an investigation into the cause of the fire.

He said anyone responsible for the blaze would be punished by the law.

Survivors were taken to nine Shanghai hospitals, and a doctor at 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.

Shanghai state television showed survivors at another hospital in the city, covered in thick blankets as they emerged from a high-pressure oxygen chamber.

Survivor Li Xiuyun, 61, told the AFP news service that she, her husband, son and a granddaughter had fled from their home on the 16th floor.

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"The smoke was very strong and the glass of the windows was scalding," she told AFP. "My son took off his socks and soaked them with water, and we used them to cover our noses. I stumbled on people on the floor when walking."

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.

Xinhua quoted local residents as saying the building was built in the 1990s and housed mainly teachers from several schools in Jing'an District, many of them retirees.

Shanghai, a city of 20 million and venue of the recently concluded World Expo, has seen a construction frenzy in recent years, ranging from high rises that dot its skyline to new subway lines, highways and airport upgrades. But unsafe building work remains a chronic problem in China.

Last year, a nearly finished 13-story apartment building in Shanghai collapsed, killing one worker. Investigations showed that excavated dirt piled next to the building may have caused the collapse.