Environmentalists say some places on the island has pollution 200% higher than annual safety guidelines.

About 400 environmentalists carried posters and chanted slogans calling for the government to do something about the territory's severe air pollution.

Many dyed their hair blue symbolising clean skies

Organiser Lisa Hopkinson said: "We want to give people some opportunity to voice frustration with the government's inaction on air pollution and also demonstrate to the administration that air pollution is a big concern of the public"

Diesel-powered vehicles, accounting for some 50% of the emissions, are mostly blamed for the poor air quality.

Most of the territory's ageing taxi fleet also runs on diesel.

Christine Loh, a legislator and march organiser, said: "If you look at cars as things that spit out poison, which they are, maybe people would maintain their vehicles in a better shape."

The territory's air pollution index has reached the "very high" band on several occasions over the past few months, prompting government warnings for people with asthma, heart and lungs ailments to avoid over-exertion and traffic-congested areas.

Environmentalists fear without radical action Hong Kong could join the ranks of the world's most polluted cities such as Bangkok or Mexico City.

George Cautherley, a working committee member of the Clear the Air pressure group, said: "The air here is terrible. It's not the worst in the world, but it is far worse than a city of this wealth should sustain."

But the authorities say that overall air quality is about "the same as many other major cities such as London and New York," and point out that many measures have been introduced to combat vehicle air pollution.

In October, the government announced proposals for vehicles to convert from diesel to liquefied petroleum gas (LPG), but activists are unhappy with the pace of the programme.