Police detained unlicensed welders on Tuesday on suspicion of accidentally starting a fire that engulfed a high-rise apartment building under renovation in China's business capital, killing at least 53 people, as public anger grew over the government's handling of the disaster.

A preliminary investigation showed that four welders improperly operated their equipment, setting off Monday's fire in Shanghai, the city government and state television reported.

Witnesses and building residents quoted earlier by state media said the blaze began when building materials caught fire. It then spread quickly to scaffolding covering the 28-story building, which houses a number of retired teachers as well as other families.

In addition to the 53 fatalities, the city government said on Tuesday that more than 70 other people had been rushed to hospitals. Deputy Mayor Shen Jun said 17 were in critical condition.

"The accident was caused by unlicensed welders improperly operating their equipment. Four suspects have been placed in criminal detention by the police," the city said on its website.

Frustration grew on Tuesday among relatives seeking answers to how such a tragedy could happen in Shanghai, a wealthy city that is one of the country's best-run urban centers.

"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 a woman who gave only her surname, Liu. She said her mother lived on the ninth floor of the building and 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," Liu said. The renovations were intended to improve the building's energy efficiency.

At one temporary facility for residents of the building, one middle-aged man was shouting that he was being stopped from going to a funeral home to identify his wife.

"I couldn't sleep last night, and have been waiting hours and hours. Why don't they tell me the truth, why don't they let me go," said the man, who refused to give his name.

The fire dominated Shanghai's skyline before it was put out after more than four hours, with black smoke billowing through the sky. The government said more than 100 fire trucks battled the blaze.

Survivors were taken to nine Shanghai hospitals, where there were sad scenes 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, searched a list of survivors 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."

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 for an investigation into the cause of the fire.

He said anyone responsible for the blaze would be punished.

Shanghai Fire Chief Chen Fei told a news conference that the fire started at about the 10th floor, and then spread quickly because the scaffolding was made of bamboo and was covered with flammable nylon nets.

"That made the fire quickly spread in vertical directions and nearby areas, forming a large-scale fire in a very short time," Chen said.

He defended his crews, saying it takes a long time to put out a high-rise fire in any country.

The official Xinhua News Agency 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 the venue of the recently concluded World Expo, has witnessed 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.

There have been no reports of serious apartment fires in China in recent years. The Shanghai fire is the worst since 53 people died in a supermarket fire in Jilin province in northeast China in 2003, according to the State Administration for Work Safety. It said 300 died in another supermarket fire in Henan province in central China in 2000.