BEIJING (Reuters) - China's capital Beijing issued an emergency pollution alert for the first time on Thursday, warning residents to reduce outdoor activities and construction sites to control dust given a forecast of heavy smog over the next three days.

Pollution is an increasing concern for China's leaders, keen to douse potential unrest as affluent city dwellers turn against a growth-at-all-costs economic model that has tainted much of the country's air, water and soil.

Severe pollution in Beijing has made it "barely suitable" for living, according to an official report published earlier this month, and despite numerous measures aimed at improving air quality in recent years, Beijing still regularly suffers from choking smog.

The Beijing Municipal Environmental Protection Bureau said on its website it had issued a Yellow alert - the first use of a color-coded warning system unveiled last October.

The warning, the second most serious in the four-color system, came even as readings of PM2.5 particles, especially bad for health, had not yet hit the more hazardous levels already seen several times earlier this year.

Just last weekend, levels of PM2.5 particles topped 500, according to a reading by the U.S. Embassy in Beijing. The U.S. Environmental Protection Agency considers a level above 300 to be hazardous.

Last weekend's lack of response by the Beijing authorities drew strong criticism in state media over the following days.

Heavy smog will continue for three days, the government warned, urging residents to use public transport if possible.

The alert also requires roads to be frequently sprayed and construction sites to control dust levels using sprinklers.

Beijing's mayor promised last month to spend 15 billion yuan ($2.4 billion) on improving air quality this year as part of an "all-out effort" to tackle pollution. The city also plans to shut 300 polluting factories and cut new car registrations.

(Reporting by Dominique Patton; Editing by Mark Heinrich)