People of Delhi, the Indian capital, were alarmed on Monday as a blanket of thick gray smog hung in the air ahead of Diwali, the festival of lights.
Visibility fell to low levels as pollution rose to a high: 20 times the World Health Organisation's recommended limit, according to the BBC.
The air quality is expected to further deteriorate over the next few days because of the use of firecrackers, according to experts.
Indian cities regularly place high on global pollution rankings for the worst air quality.
The Supreme Court has restricted the timeframe for setting off firecrackers to only two hours during the day – but experts believe this embargo will be difficult to enforce.
Diwali, the most important Hindu festival in north India, is a celebration of the victory of good over evil.
However, in the last few years, celebrations have seen air pollution rise to dangerous levels after firecrackers were set off.
Many have taken to social media to share their concerns and post photos of the city to show the extent of the pollution.
Authorities in Delhi have urged residents to stay indoors as much as possible and have started to pause construction activity in the city.
The Indian capital is the sixth worst place in the world for pollution, according to World Health Organisation (WHO) data.
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