Competitors at Beijing International Marathon don face masks to battle heavy pollution

A runner, wearing a mask to protect herself from pollutants, looks at her smartphone as she and others jog past Chang'an Avenue near Tiananmen Square shrouded in haze at the start of 2014 Beijing International Marathon in Beijing, China Sunday, Oct. 19, 2014. (AP Photo/Andy Wong) (The Associated Press)

A runner, wearing a mask to protect himself from pollutants, jogs past Tiananmen Gate shrouded in haze while taking part in the 2014 Beijing International Marathon in Beijing, China Sunday, Oct. 19, 2014. (AP Photo/Andy Wong) (The Associated Press)

Runners jog past Chang'an Avenue near Tiananmen Square shrouded in haze at the start of 2014 Beijing International Marathon in Beijing, China Sunday, Oct. 19, 2014. (AP Photo/Andy Wong) (The Associated Press)

An international marathon has gone ahead in China's capital despite heavy pollution over the city.

The 34th Beijing International Marathon began Sunday morning at Tiananmen Square with many of the tens of thousands of competitors wearing face masks.

An update Saturday night on the marathon's official microblog said "there might be slight or moderate smog" on Sunday. It asked competitors to pay attention to the conditions and to take measures according to their own health, and advised elderly athletes and those with respiratory diseases to carefully consider whether to participate.

The Beijing News reported that postponing the marathon would be difficult because of the huge organization that had gone into it.

About 30,000 people were expected to take part in the marathon and the half-marathon, according to the Beijing News.