A grey haze descended on the city of around 21.5 million people with levels of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- at one point above 300 micrograms per cubic metre according to the US embassy which issues independent readings.

The World Health Organization's recommended maximum exposure is 25.

The alert 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.

It was the first time authorities declared a "red alert" since emergency air pollution plans were introduced two years ago although levels were far from the city's worst.

It came a week after thick grey smog shrouded Beijing cutting visibility severely and sending PM 2.5 levels as high as 634 micrograms per cubic metre.

Under the alert -- the highest in a four-tiered colour-coded warning system -- an odd-even number plate system bans half the city's roughly 4.4 million private cars from the streets on alternate days.

Outdoor construction sites are ordered to close and some industrial plants told to cease or reduce operations.

Some schools are also urged to close and several Beijing residents said their children had been told to stay at home.

Authorities in the capital were heavily criticised after only issuing an orange alert for last week's pollution.

"The red alert is a welcome sign of a different attitude from the Beijing government" said Dong Liansai climate and energy campaigner for environmental group Greenpeace.

"However this the latest of a series of airpocalypses to hit Beijing is also a firm reminder of just how much more needs to be done to ensure safe air for all."

Nonetheless some social media commenters said the measures were not enough.

"Can we apply to work at home? The air in our office is totally 'poison gas'" said one poster on Sina Weibo a Chinese version of Twitter.

The decision to issue the red alert despite relatively low numbers also provoked ridicule.

"Today wasn't as serious as the previous time" said one commenter. "How could they not issue a red alert then and issue a red alert now?"

An editorial in the government-published newspaper the China Daily on Tuesday said the move showed that "authorities have listened to residents' concerns."

"Of course" it added "we don't expect the frequent issuance of a red alert and we hope that we will be able to forget about it in the near future" when the government can "keep the air clean for good".