Dense gray smog shrouded the roads of the world's most polluted capital, where many pedestrians and bikers wore masks or covered their mouths with handkerchiefs and scarves.
The US embassy website showed the concentration of PM 2.5 -- the microscopic particles that are the most damaging to health -- topped 700 early on Wednesday morning, 70 times the WHO guidelines on long-term exposure, before dropping slightly.
"When I came to Delhi in 1984, the air in the city was clean. But today when I left at 4 am for work I could barely see anything," said Jeevanand Joshi, a roadside tea seller.
"This is not fog, this is smoke, and it is certainly making us sick."
The Indian Medical Association declared a public health emergency, urging administrators to "curb this menace", while the Environment Pollution Authority warned that things were set to get worse in the coming days.
As public outrage mounted, the Delhi government ordered the closure of all primary schools on Wednesday.
"We have decided to shut schools up to primary level for a day, and will evaluate the situation on an hourly basis to see if such a closure needs to be extended," Delhi's deputy chief minister Manish Sisodia told reporters.
Almost 2 million students are enrolled in primary schools in Delhi, according to government data from 2015.
All outdoor activities have also been banned across the capital's 6,000 schools while pollution levels remain at severe levels.
The city of 20 million has the unenviable distinction of being the world's most polluted major city, often surpassing Beijing.
Since 2014, when WHO figures showed the extent of the crisis, authorities in Delhi have closed power plants temporarily and experimented with taking some cars off the road.
But the temporary measures have so far had little effect.
Delhi's air quality typically worsens ahead of the onset of winter as cooler air traps pollutants near the ground, preventing them from dispersing into the atmosphere, a phenomenon known as inversion.
High levels of moisture in the air and a lack of wind meant emissions had become trapped in the environment, according to India's Central Pollution Control Board.
Firecrackers set off to celebrate the Diwali festival of lights in the city add to the toxic mix created by pollution from diesel engines, coal-fired power plants and industrial emissions.
The problem is further exacerbated by the burning of crop stubble by farmers after the harvest in northern India, a practice that remains commonplace despite an official ban.
On Tuesday India's Environment Pollution Authority, which was set up by the Supreme Court to tackle the issue, ordered the closure of dust-spewing brick kilns and an increase in parking fees to encourage the use of public transport.
"In terms of air pollution, things are expected to get much worse in the coming days," Bhure Lal, head of the agency, said in a statement late Tuesday.