Excavators flanked by Bangladesh riot police are at work demolishing illegal soot-belching brick kilns around the smog-choked capital Dhaka, forcing migrant labourers out of work and back to their villages.

Every autumn, following the monsoon rains, Dhaka's brick kilns — which use coal and wood to fire bricks from clay — start up again, adding to the emissions pumped out by other heavy industries and the thousands of vehicles on the streets of the capital.

On November 25, an independent air quality monitor pegged Dhaka's air as the most polluted in the world. The next day, the high court ordered the hundreds of illegal brick factories that surround the city to be closed within two weeks.

Many were built in the past five years as heavy industry and construction fuelled a booming economy.

While authorities say tearing them down will make Dhaka's air more breathable, thousands of kiln workers — who hail from poor rural regions or coastal areas hit by climate change — have been left without a job.

Standing beside an excavator as its metal teeth bit into a tall kiln chimney at Saturia, west of the city, magistrate Kazi Tamzid Ahmed ordered police to keep the workers at bay.

"It (the brick kiln) flouted environmental regulations [...] It is also set up near a school," he told AFP.

The kiln's owner Nazrul Islam Nabin pleaded tearfully for the excavator to be stopped, but to no avail. Some 300 workers were now without a job and would have to head home to their villages on the south coast, he said.

"We sought 15 more days from the authorities, saying we'll pay off the dues of the workers by selling bricks. But they didn't heed our call," he said.

Most workers travel to urban brick kilns during the winter months, where they earn between 300-800 Taka ($3.5 - $9.5) per day, shovelling coal into furnaces or laying brick out to dry in the sun. The money they save keeps them and their families afloat for the rest of the year.

Almost half of the 7,000 kilns across the country are illegal, Bangladesh Brickfield Owners Association secretary Abu Bakar told AFP, employing almost one million people.

The campaign so far has closed at least 25 illegal kilns, Rubina Ferdowshy, the environment department's director told AFP.

The demolitions have "improved Dhaka's air quality," she said. "We now rank much below among the worst polluted cities."

By early December Dhaka's air had improved, coming in at 23rd worst among major world cities according to the same monitor.

But for Bishwanath Mallick, who used to work at the Saturia kiln, the improved ranking has come at a price.

"Now, where will I find work? There are only shrimp farms in my village, but they don't need many workers," he told AFP.