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.