Visibility was only 25 metres in some areas, affecting traffic movement. Poor visibility in Palam, near the city’s international airport, delayed a dozen flights.
The airport authority and airlines advised passengers of dense fog that might affect flight operations, and asked them to contact airlines for flight information.
The dense fog also affected the country’s railway network, which carries about 23 million passengers daily.
As many as 30 trains were running late in northern India on Monday.
Homeless people, among the most affected by the drop in temperature, could be seen huddled on street corners burning large bonfires to keep themselves warm.
The Delhi government has extended winter school holidays until January 15.
India's weather department declares a cold wave when the minimum temperature is less than 4°C.
The city recorded 1.9°C on Sunday — the second-lowest minimum temperature since 2008.
Delhi’s neighbouring states have also been reeling under severe cold. Thick fog has extended from northern Punjab, adjoining north-west Rajasthan, Haryana, and Uttar Pradesh and eastern Bihar.
Punjab's Bhatinda town had zero visibility due to dense fog.
The India Meteorological Department has issued an “orange” alert for certain parts of north India, including Delhi, for at least two days.
“Due to the continuation of prevailing light winds and high moisture near the surface over the Indo-Gangetic plains, dense to very dense fog is very likely to continue in some parts during the night and morning hours over Delhi during the next two days and dense fog in isolated pockets thereafter for the subsequent three days over the region,” it said.
Several other northern states have announced school holidays for their students.