Runners were forced to wear face masks as tens of thousands of competitors took part in an international marathon in Beijing under a thick blanket of smog – despite warnings that everyone in the city should avoid outdoor activities.

About 30,000 runners were expected to take part in the event on Sunday morning, with the organising committee making 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.

“On a normal day, nobody would run in such conditions,” said participant Liu Zhenyu, a computer engineer. “But the event is happening today, so what can we do?”

Although organisers had warned on Saturday night that “there might be slight or moderate smog“, the air was deemed to be severely polluted on Sunday morning, according to the real-time monitoring of Beijing’s environmental centre.

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.

According to the Beijing News, the marathon’s organizing committee said on Saturday night that postponing the event would be difficult because of all the planning that had gone into it.

The US 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 in smog.