Buildings neighboring the Summer Palace are visible under the clear blue sky in Beijing on Aug 22, 2015. [Photo/CFP]

Air quality in Beijing is expected to remain at the optimum standard in the coming days, as heavily polluting factories have curbed or stopped production before the World War II victory anniversary parade on Sept 3, environmental authorities said on Monday.

"In all, 1,927 factories in Beijing were required to reduce air polluting emissions by at least 30 percent starting on Aug 20," said Mu Liang, head of a monitoring team at the municipal environmental monitoring center.

"Our teams are conducting random inspections round-the-clock these days, more frequently than usual."

Offenders failing to meet emission standards in random inspections must stop production or face fines up to 500,000 yuan ($78,300), the Beijing Environmental Protection Bureau said in a statement.

"Regulations are not solely based upon factories' emission-curbing plans approved by the monitoring center," Mu added. "Once emissions are found to be exceeding permissible limits in real-time monitoring, factories are required to adjust production immediately."

During a random inspection of some of the most polluting businesses in Pinggu district on Monday, a China Resources Snow Breweries factory, one of the largest beer producers in China, was ordered to use a lower-powered coal-fired boiler for beer fermentation over the next few days.

"The company did not break the rules in that one of its two boilers had been stopped, according to its plan," said Zhang Yu, engineer at the monitoring center, "but one of three major pollutants failed to meet the standards."

Nitrogen oxide had exceeded the benchmark by nearly 50 percent - "way too high to continue as planned", Zhang added.

"We understand how reducing production could impact revenues, especially during the peak season for beer consumption in the summer," said Mu, "but we have to guarantee that curbing measures are going well to ensure clean air for the military parade."

Wang Yanfei contributed to this story.