BEIJING - Schools closed and rush-hour roads were much quieter than normal as Beijing invoked its first-ever red alert for smog Tuesday, closing many factories and imposing restrictions to keep half the city's vehicles off the roads.

The alert in effect through Thursday - the most serious warning on a four-tier system adopted in 2013 - means authorities have forecast three consecutive days of severe smog.

Despite improvement in Beijing's air over the last year, readings of dangerous particles Tuesday were as high as 14 times the safe level, in what has become an embarrassment for a government that has made a priority of cleaning up after years of full-tilt economic growth.

A soupy gray haze subsumed Beijing's unique landmarks, and convenience stores did brisker-than-usual business selling air-filtering masks.

"This is modern life for Beijing people," office worker Cao Yong said. "We wanted to develop, and now we pay the price."

Under the alert, schools were advised to voluntarily close unless they had good air-filtration systems. Beijing's education commission later issued a separate order for all schools to close through Thursday.

Along with limiting cars to driving every other day depending on license-plate numbers, other restrictions will seek to reduce the amount of dust and other particulate matter in the city of 22.5 million. While pollution in the capital improved in the first 10 months of the year, heavy smog that can be seen from outer space can even prompt highway closures because of reduced visibility. Much of the pollution is blamed on coal-fired power plants.