By Tanvi Mehta NEW DELHI (Reuters) -Toxic smog engulfed much of northern India on Monday and pollution readings in New Delhi hit their highest of this pre-winter season, after dense fog overnight. Visibility dropped to 100 metres in the capital and at least eleven flights were diverted.

In several areas, pollution levels were more than 50 times higher than the World Health Organization's recommended safe limit.

A thick blanket of toxic smog engulfed most parts of northern India on Monday and readings of air quality in the capital New Delhi hit their highest this year after dense fog overnight.

Delhi's air quality plunged to hazardous levels on Monday, recording a season-high AQI of 493. Strict Stage 4 curbs under GRAP, including truck bans and halted public construction, are enforced in the capital city.

Delhi recorded air pollution more than 17 times above an acceptable level as worsening smog triggered curbs on trucking and shuttered construction sites and schools.

India said on Sunday that it would tighten restrictions including on construction activities and vehicle movements in New Delhi and surrounding areas from Monday to combat worsening air quality. The new measures,

Pollution in New Delhi shot up to its worst level of the year so far, prompting local authorities to enact emergency measures, including school shutdowns and a ban on construction work.

India's capital New Delhi switched schools to online classes Monday until further notice as worsening toxic smog surged past 60 times the World Health Organization's recommended daily maximum.- 'Stay indoors' - Primary schools were ordered to cease in-person classes on Thursday,

India's capital New Delhi switched schools to online classes Monday until further notice because of worsening toxic smog, the latest bid to ease the sprawling megacity's health crisis.