All junior schools in New Delhi were closed on Wednesday and health warnings were issued for the sick and elderly as pollution remained at hazardously high levels for a second day.
The air quality index that measures the concentration of poisonous particulate matter in the air was at a "severe" level of 471, where the maximum reading is 500 and anything above 400 is considered unhealthy by the Central Pollution Control Board.
A thick brown smog hung over the city with residents reporting smarting in the eyes and irritation in the nose and throat.
Following Delhi government orders, primary schools were closed and no outdoor activities including morning assemblies were permitted for older pupils.
As the Indian Medical Association declared a public health emergency, authorities issued an advisory for "high-risk" groups like the elderly, heart and asthma patients to take care and avoid morning or evening walks.
Domestic media reported the level of pollution was 25 times the international safe limits, in certain areas; equal to smoking 50 cigarettes a day.
Delhi Chief Minister Arvind Kejriwal said the hazardous air pollution was due to crop stubble burning by farmers in neighbouring states and he had contacted authorities to address the situation.
Experts expect the next few days to be as bad since calm wind conditions and high humidity were leading to an accumulation of pollutants.
Delhi has been ranked among the top polluted cities in the world in recent years, but efforts to check the worsening air quality have not worked so far.
An Indian woman and children walk amid heavy smog in New Delhi on Wednesday.
Indian visitors walk through the courtyard of Jama Masjid amid heavy smog in New Delhi on Wednesday
Schoolchildren cover their faces as they walk to school amid heavy smog in New Delhi
A schoolboy walks along a road amid heavy smog. Delhi shut all primary schools on Wednesday
People commute on a smoggy morning in New Delhi on Wednesday
People ride a boat as seagulls fly over the waters of the river Yamuna amid smog in New Delhi
A policeman covers his face with handkerchief as he walks during smog in the Indian capital
People walk on a street as smog shrouded the roads of the world\'s most polluted capital
Commuters drive on a road in New Delhi, where many pedestrians and bikers wore masks