A blinding layer of fog shrouded the Indo-Gangetic plains in North India on Sunday, with visibility levels plunging to zero metres at several places.
The India Meteorological Department (IMD) has advised people to avoid unnecessary travel and to take precautions while driving.
A spokesperson for the Railways said fog impacted the schedules of "22 trains arriving in Delhi."
Satellite imagery showed a layer of dense fog extending from Punjab and north Rajasthan to the northeast. Patches of fog were also visible along the east coast.
An IMD official said dense fog engulfed Punjab, Haryana, Delhi, North Rajasthan, and Uttar Pradesh starting at 10 pm on Saturday.
"This is the first time this winter season that zero-metre visibility was reported from Amritsar to Dibrugarh across Ganganagar, Patiala, Ambala, Chandigarh, Delhi, Bareilly, Lucknow, Bahraich, Varanasi, Prayagraj, and Tezpur (Assam)," the scientist said.
"It has been the longest duration of dense fog this season. Also, it is the most intense so far," he added.
Commuters should be extremely careful while driving on highways and use fog lights, the IMD said.
At 5:30 am, visibility levels stood at 25 metres in Patiala, Ambala, Bahraich (UP), Purnia (Bihar), and Palam (Delhi), and 50 metres in Amritsar, Chandigarh, Safdarjung (Delhi), Bareilly, Lucknow, Varanasi, Dibrugarh, and Tezpur (Assam).
Guwahati (Assam), Kailashahar, and Agartala (Tripura) reported a visibility level of 200 metres.
The Palam Observatory near the Indira Gandhi International Airport in Delhi reported dense fog with visibility levels plunging to zero metres by 5 am.