Pollution levels on Friday were among the highest ever recorded. 2013 Getty Images

Local residents wait on motorcycles and bicycles at a traffic junction in downtown Shanghai. Aly Song/Reuters

An aircraft sits in thick smog on the tarmac of Hongqiao airport in Shanghai on Friday. 2013 AFP

A tourist wearing a face mask visits the promenade in Shanghai, as hazardous levels of air pollution invade the city. Xiao Junwei/AP

Hundreds of flights were delayed or canceled Friday in Shanghai as record levels of air pollution shrouded China's commercial hub in smog, prompting authorities to urge residents to stay indoors.

On Friday afternoon, the Shanghai government issued its most severe health warning as the city's pollution index ranged between 23 times and 31 times the levels recommended by international health officials.

In the first such advice since a new health warning system was launched in April, authorities urged residents to stay indoors and asked factories to either cut or halt production.

Most of the flights leaving Shanghai's Pudong International Airport, one of the world’s busiest cargo airports, were delayed due to decreased visibility, according to the airport's website.

Hazardous air pollution forced schools to shut or suspend outdoor activities in at least two other cities in eastern China on Thursday. Some schools in Shanghai canceled outdoor activities on Friday, as well.