One of China's biggest cities has imposed curbs on traffic and factories due to a spike in air pollution and told elderly people and children to stay indoors.

The government of Tianjin, a port city of 15.5 million people east of Beijing, said half its cars will be barred from the road each day starting Sunday based on whether their license plate numbers are odd or even.

Factories were ordered to reduce emissions by 50 per cent.

Kindergartens and primary schools were told to cancel outdoor activities.

China's major cities are among the world's smoggiest and regularly suffer pollution spikes that prompt emergency measures.