Despite heavy pollution blanketing Beijing on Sunday, an international marathon went ahead, with face masks and sponges among the equipment used by competitors to battle the smog.

The 34th Beijing International Marathon began at Tiananmen Square with many of the tens of thousands of participants wearing face masks. The 26-mile course ended at the Chinese capital's Olympic Park.

An update Saturday night on the official microblog of the marathon, which is hosted by the Chinese Athletic Association and the Beijing Municipal Bureau of Sports, said "there might be slight or moderate smog." It asked competitors to pay attention to the conditions and to take measures according to their own health, and advised the elderly and people with respiratory diseases to carefully consider whether to participate.

About 30,000 people were expected to take part in the marathon and the half-marathon. The organizing committee made 140,000 sponges available at supply stations along the marathon route so runners could "clean their skin that is exposed to the air," the Beijing News reported.

The air Sunday was deemed severely polluted, according to the real-time monitoring of Beijing's environmental center. It was the most serious level on China's air quality index, and came with a warning for children, the elderly and the sick to stay indoors, and for everyone to avoid outdoor activities.

The U.S. Embassy, which tracks the Beijing air from a monitoring station on its roof and uses a different air quality index, said the air was hazardous. It gave a reading of 344 micrograms per cubic meter of PM2.5 particulate matter. The World Health Organization considers 25 micrograms within a 24-hour period a safe level.

China's pollution is notorious following years of rapid economic development. Combating the problem has shot up the agenda of the ruling Communist Party, which is under pressure from citizens who are tired of breathing smog.

The marathon's organizing committee said late Saturday that postponing the event would be difficult because of all the planning that had gone into it, and asked competitors to understand, the Beijing News reported. It said 46 percent of the competitors had traveled from abroad and other parts of China to take part.