BEIJING » Schools in Beijing were ordered to keep students indoors Tuesday after record-breaking air pollution in the Chinese capital soared to up to 35 times the safety levels.

The pollution spike is a reminder of China’s severe environmental challenges as President Xi Jinping joins other world leaders at the Paris climate conference.

Factories and construction sites were told to reduce work after the city government on Sunday issued its first orange alert — the second highest of four warning levels — in almost two years.

On Monday, concentrations of airborne particles smaller than 2.5 microns in diameter, or PM2.5, peaked at 900 micrograms per cubic meter in southern Beijing. Such particles are especially damaging to lung tissue. The World Health Organization’s recommended maximum is 25 micrograms per cubic meter.

Visibility fell to several hundred meters (yards), leaving buildings silhouetted in the haze. People complained of a smoky, pungent odor and many wore tight-fitting face masks.

“I felt like my lungs were blocked,” said Xu Pengfei, a security guard at a downtown office building. “We have to stand in the open for many hours a day, and the pollution really affects us.”

China’s cities are among the world’s dirtiest after three decades of explosive economic growth that led to construction of hundreds of coal-fired power plants and the spread of automobile ownership.

Communist leaders have tightened emissions standards and are investing in solar, wind and other renewable energy. But the country still depends on coal for more than 60 percent of its power.

Tests found coal burning to be to blame for the bulk of the latest pollution surge, the official Xinhua News Agency said, citing Zhang Dawei, head of the city’s environmental monitoring center.

Power demand soared due to unusually cold weather in November. For most of that month, the capital was shrouded in persistent smog.

Air quality worsened on Friday and deteriorated throughout the weekend. Authorities said they avoided issuing the highest-level alert because conditions were forecast to improve by Wednesday.

Beijing schools were ordered to stop outdoor activities. A primary school in the Xicheng district on the west side sent a message to parents that classes were canceled Tuesday.

Conditions were worsened by cold air that trapped pollutants near the ground, according to Zhang, the environment official. He said pollution from surrounding areas also blew into the capital.

Outside Beijing, readings for PM2.5 were was as high as 976 micrograms in the suburban region of Liulihe.

Several cities in the northern province of Hebei, which surrounds Beijing, also reported extremely polluted air.

Inspectors from the Ministry of Environmental Protection found some construction projects flouted orders Monday to stop work that could raise dust, according to Xinhua.

Outside Beijing, reduced visibility due to heavy fog prompted authorities to close 1,553 highway sections in central, eastern and southern China, the Transportation Ministry said on its website.

AP news assistant Dong Tongjian contributed to this report.