Much of the city was shrouded under thick smog
Air pollution levels in Hong Kong have reached a record high, prompting government warnings to people to avoid going out.
The Environmental Protection Department said some Air Pollution Index readings were more than double the level at which people should stay indoors.
Some schools stopped children playing outside to safeguard their health.
The record levels follow severe sandstorms thousands of kilometres to the north around the Chinese capital.
Officials said the sandstorms had exacerbated Hong Kong's worsening smog problems.
A spokeswoman for Hong Kong's environmental agency said that the API - a ratio based on the concentration of pollutants in the air, including sulphur dioxide and lead - was at "record high levels".
People with heart or respiratory problems are advised to stay indoors at an API reading of more than 100; the public is advised to stay indoors at more than 200.
On Monday the API was 453 at one recording station with five other stations marking levels of above 400, the government said.
"As the sandstorm from northern China is moving southward with the northeast monsoon and is now affecting Hong Kong, the Air Pollution Index is expected to reach the 'very high' or 'severe' level," the government said in a statement.
"Hong Kong's air pollution is bad already, but this shows we're not dealing very well with the most severe weather situations. It is a very big alarm," Edwin Lau, director of Friends of the Earth Hong Kong, told AFP.
The government has warned the public to avoid prolonged exposure to heavy traffic areas and to reduce physical exertion and outdoor activities.
Schools were told to cancel sporting activities; elderly people have sought emergency help for shortness of breath.
The Clean Air Network recently helped to launch a business lobby to urge more government action against pollution.
The BBC's correspondent in Hong Kong, Annemarie Evans, says there are are tens of thousands of factories across the border in southern China which adversely affect Hong Kong's air quality, but that roadside pollution remains a large part of the problem.