On Sunday, about 700 protesters braved temperatures of -20 degrees Celsius (-4 degrees Fahrenheit) to demand cleaner air. They held banners reading "We want clean skies!" and "Stop poisoning our children!"

Low winds helped the thick grey layer of smog called "black sky" by the locals settle over the city of 1.2 million. The Chelyabinsk meteorological office reported that air pollution exceeded healthy levels before the smog dissipated.

"The ecological situation in the city is getting worse and worse, and the government does not want to take measures to correct it," protester Artur Abuzarov said during Sunday's rally.

The protest followed an earlier one in which participants tried unsuccessfully to force their way into the mayor's office. The situation went unreported in state-controlled media.

Locals say the pollution problem has persisted for years and they fear air quality will worsen further if a new copper mining enterprise opens next year as planned.

"We are suffocating, children are ill, the ecology is bad," said another demonstrator, Tatyana Pominova. "What is happening in Chelyabinsk is a complete disgrace. It is impossible to breathe and it is impossible to live."

Anastasia Zubareva, a doctor who specializes in conditions of the ear, nose and throat, attributed the city's high number of childhood illnesses to air pollution, noting that her patients only feel better when smog dissipates.

And it's not only children. Galina Gurina, 58, attributes her chronic headaches and asthma to industrial emissions.

Chelyabinsk officials have dismissed the residents' protests.

Vitaly Bezrukov, a local official who deals with environmental issues, acknowledged that power plants and other sites sporadically produce pollutants in concentrations that exceed permissible levels. But he said they do not pose a danger for people.