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.

Copyright Agence France-Presse, 2019