Young and old residents of the Chinese metropolis of Wuhan were advised to stay indoors on Monday after a thick haze blanketed the city of nine million people, official media said.
Described by residents as opaque with yellowish and greenish tinges, the fug descended suddenly in the morning, prompting people to rush to put on face masks, witnesses told AFP.
The official Xinhua news agency quoted the environmental protection department of Hubei province saying in a statement: "Children, the elderly and people with heart or respiratory diseases are advised to stay indoors."
Xinhua said straw burning was the cause and denied there had been any industrial accidents in or near Wuhan, after Internet rumours suggested there had been an explosion at a chemical complex northeast of the city.
"I looked out of the window of my office and I could not believe my eyes," said resident Li Yunzhong.
"At first I thought it was going to rain. In 31 years in Wuhan I have never known anything like it. We are very worried because we do not know what it is."
France's consulate-general in the central city advised residents to stay at home, close their windows and limit the use of air-conditioning.
"The source of the thick cloud that has covered the city of Wuhan since this morning is at present unknown," it said on its website.
"Local authorities have promised us the information as soon as possible."
Xinhua described the haze as grey-yellow in colour and said it was seen in seven cities in Hubei province, including Wuhan.
Air pollution is increasingly acute in major Chinese cities and authorities are frequently accused of underestimating the severity of the problem in urban areas, especially in Beijing.
Air-quality monitoring showed Wuhan's PM10 particulate concentration stood at 0.574 mg per cubic metre at 2:00 pm, more than triple the daily average of 0.150 mg, Xinhua reported.
But it quoted the environmental protection department saying industrial accidents were not responsible and analysis showed an increase in carbon particles from burning organic matter.
"Many farmers choose to burn crops that are left behind in their fields after harvesting," Xinhua said.
But Li was sceptical. "I doubt that," he said. "We don't practise large-scale shifting agriculture in our region."
Another resident told AFP she was leaving the city because of the cloud.
Wuhan is the capital of Hubei province and an industrial centre where many foreign firms have set up factories, including the French automotive group PSA Peugeot Citroën.
Alstom also manufactures boilers for coal-fired power plants there.
China's environment suffers from industrial pollution, increasing traffic and lax protection measures.
Official air-quality statistics are sometimes at odds with non-government measurements, and are often viewed with distrust.