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, toxic smog has enveloped India's capital, home to more than 33 million people. One technology firm which monitors air pollution ranked New Delhi as the most polluted city in the world.

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.

Toxic smog continues to plague most of northern India, including the capital, New Delhi. With levels of microparticles well over recommended limits in the city, schools have been ordered to hold classes online.

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,

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.

The Supreme Court agreed for an urgent hearing of a plea on Monday seeking directions to curb air pollution in the national capital and adjoining areas.