Diesel generators and a Delhi power plant were shut down on Tuesday as air quality in the capital plunged ahead of Diwali.

The Environment Pollution (Prevention and Control) Authority (EPCA) made the order as the city's air hit the 'very poor and severe' category ahead of Thursday's festival of light, which is expected to send toxicity in the air rocketing with a night of fireworks.

Some brick kilns were also ordered to close and the burning of rubbish to be stopped under the plan that imposes anti-pollution measures when air quality drops below a certain level.

Fireworks at the Diwali festival of lights, which sends toxic air levels rocketing (file pic)

The measures will stay in place until March to prevent pollution levels from rising.

Many richer residents of Delhi have diesel generators as a back-up during regular power supply disruptions.

Emergency services like hospitals will not be forced to shut down their generators.

'Delhi has a long way to go before it can lay claim to having reasonably clean and breathable air,' Sunita Narain, an EPCA member and director general of the Centre for Science and Environment, a Delhi-based thinktank, said in a statement.

School children going back to home after their school was closed due to smog after last year's celebrations

Air pollution in Delhi has become a major public policy issue in recent years as residents wake up to the life-threatening implications of breathing air frequently ranked among the world's most toxic.

Indian courts have taken up much of the role in trying to clean the air, although enforcement of the various bans and restrictions has proven difficult.

Last week the Supreme Court temporarily banned the sale of firecrackers in and around Delhi ahead of Diwali, the Hindu festival of lights, as it looks to prevent a repeat of severe air pollution that forced school closures last year.

Indian school children wear masks as they sit inside a school cab as schools re-open after three days of closure due to smog following Diwali 2016

Last November, about a million children were forced to stay home from school and thousands of workers reported sick as New Delhi struggled with its worst pollution for nearly 20 years.

Vehicle emissions and dust from construction sites were the factors blamed for that spike, besides firecrackers and crop burnings.

India and China together accounted for more than half of the 4.2 million deaths attributable to air pollution worldwide in 2015, a study by the U.S.-based Health Effects Institute showed.

The closure of Delhi's Badarpur thermal power plant is one of the actions required when the air quality turns very poor by the government's Graded Response Action Plan (GRAP), which came into force in January.

Power plants in neighbouring states will also be monitored for pollution levels.