China's capital issued its first-ever red alert for pollution on Monday, as a new blanket of choking smog was projected to descend on the city.

From Tuesday morning half of Beijing's private cars will be ordered off the road, with an odd-even number plate system in force, and 30 percent of government vehicles also garaged.

Outdoor construction sites will cease operations, but only some industrial plants will have to "implement measures to limit or stop production", said Beijing's Environmental Protection Bureau on its verified social media account, adding that fireworks and barbecues were also banned.

"People should to the best of their ability reduce outdoor activities," it said. "If you are engaging in outdoor activities you should wear a mask or take other protective measures."

Kindergartens, primary and middle schools were urged to close, it added, without explicitly making the measures mandatory.

Chinese social media users deemed the government's protection measures lacklustre.

"If you don't give us a day off school, what's the point of even talking about this?" asked one.

Many railed that the measures inconvenienced people without dealing with the real sources of smog, with one user saying: "Those big trucks with excessive emissions, steel plants of excess capacity, and coal mines -- going after any one of those would be better than fixating on cars if you really want to fix the haze; that's useless."

A red alert, issued when severe smog is expected to last more than 72 hours, is the highest of Beijing's four-tiered, colour-coded warning system.

The capital has never issued it since the adoption of an emergency response programme for air pollution in 2013, despite frequent bouts of serious smog.

Monday's red alert came just a week after a thick grey haze shrouded the city with concentrations of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- as high as 634 micrograms per cubic metre.

The reading given by the US embassy dwarfed the maximum recommended by the World Health Organisation, which is just 25 micrograms per cubic metre.

It also coincided with global climate change talks in Paris, where Chinese President Xi Jinping has vowed "action" on greenhouse gas emissions.

Most of China's greenhouse gas emissions come from the burning of coal for electricity and heating, which spikes when demand peaks in winter and is the main cause of smog.

The issue is a source of enduring public anger in China, which has seen breakneck economic growth in recent decades but at the cost of widespread environmental damage.

- 'Gas chamber' -

On Monday evening, Beijing PM2.5 levels were 206 micrograms per cubic metre according to the US embassy, and 187 according to local authorities, with visibility significantly better than the previous week.

"If today is a red alert, then what was it I was seeing last week?" asked one incredulous user.

Another felt too battered from the last round of smog to feel consoled by the city's upgraded alert: "I'm already indifferent, it's all a gas chamber anyway."

Pollution is blamed for causing hundreds of thousands of premature deaths every year.

China is estimated to have emitted nearly twice as much carbon dioxide as the United States in 2013, and around two and a half times the European Union's total.

Beijing has pledged that emissions will peak "around 2030", without saying at what level and implying several years of further increases.

It has promised to reduce coal consumption by 100 million tonnes by 2020 -- a small fraction of the 4.2 billion tonnes it consumed in 2012 -- and cut 60 percent of "major pollutants" from coal-fired power plants, without specifying the chemicals in question.