The block was being renovated to improve energy efficiency.
A preliminary investigation showed four welders improperly operated their equipment, sparking the blaze in Shanghai, the city government said. Police confirmed eight people had been detained.
Shanghai's fire chief said the blaze started on the tenth floor and spread quickly to scaffolding and nylon nets covering the 28-storey building. The inferno sent black smoke billowing across the city's skyline.
In addition to the 53 fatalities, the city government said more than 70 other people had been taken to hospitals. Fifteen mostly elderly people were in serious condition, suffering from smoke inhalation, the deputy director of the Shanghai health bureau, Li Weiping, said.
Frustration grew yesterday among relatives seeking answers to how such a tragedy could happen in one of the country's best-run urban centres.
"It is hard to believe the government now. The drills on TV are successful, but when a fire truly happens, it's just useless. We feel helpless," said one woman who gave only her surname, Liu. She said her mother died in the fire.
"There must be something illegal in the construction materials, though we don't know. I am waiting for the government's explanation," she said.
Chen Jiulong, Shanghai's deputy police chief, said: "This fire is a man-made disaster involving heavy responsibility and we must pursue those who are legally responsible for it."
Shanghai fire chief Chen Fei said 200 firefighters went into the burning building and rescued 107 people. He said once the scaffolding and nylon nets caught fire the flames spread quickly, fanned by high winds.