Residents of Delhi have awoken to a toxic smog cloud after a night of Diwali celebrations in which rules restricting the use of fireworks were widely ignored.

The US embassy tweeted that levels of small air pollutants (PM 2.5) hit 689 in its part of the capital, while AirVisual, which monitors air pollution around the world, said levels hit 980 in one part of the city at 4.30am.

Levels of 50 or lower are defined as safe, while prolonged exposure to 300 or more is considered hazardous to human health.

Despite that, most Delhiites went about their business on Thursday morning as usual and unprotected by face masks, the use of which has not caught on here as it has in major Chinese metropolises.

The Supreme Court of India had ordered that only “green” fireworks be used to celebrate Diwali on Wednesday night, and that they only be set off between 8pm and 10pm.

But question marks remained over what actually constituted a “green” firework and where they could be bought, and the blast of rockets and bang of crackers could be heard across the city for hours both before and after the allotted window.

As a result, pollution levels were about the same on Thursday morning as at the same time in 2017, when PM 2.5 levels hit around 1,000 – roughly 10 times worse than the average in Beijing at the time.

“This is not good,” said Mohammad Shahid Raza, a young student living in south Delhi who said he was kept up by fireworks until 1am. “It was very noisy, and on top of that the pollution level was so high, that when I went outside my house to buy some water, I could see dark clouds, and everything was completely dark.”

For a second year, Delhi’s chief minister has likened the city to a “gas chamber”. Arvind Kejriwal hit out at the failure of neighbouring states to prevent the burning of crop stubble in the lead up to Diwali, saying that “as a result, the farmers will suffer on one hand and Delhi will become a gas chamber soon”.

A task-force under the federal pollution control board was scheduled to meet at 11am to assess the situation, a city government spokesman told Reuters.

Many citizens took to social media to express their exasperation with the widespread use of fireworks, even within the allotted time window.

Police had said plainclothes officers would be out on the streets to ensure the Supreme Court restrictions were met. But in a city of more than 20 million, enforcement seemed all but impossible.

And authorities generally have been reluctant to take stricter measures against fireworks, seen by so many as an essential element of enjoying Diwali, for fear of offending the country’s majority Hindu community.

Measures have been taken this week to try to reduce dust and pollution levels from inside Delhi itself, with water sprinkled on roads in many neighbourhoods and construction projects halted.