In the video, the woman advances on the police officer, tussling with him while holding her child to her chest. The officer remonstrates her, and, after a momentary lull, she advances again, pushing and prodding him.

Then, 22 seconds into the video, as a colleague opens the door to a police vehicle, the officer suddenly slams the woman to the ground, sending the infant flying to the pavement. The officers wrestle the woman on the ground, initially oblivious to the child, who is helped by onlookers.

The confrontation has distilled two concerns close to the heart of many Chinese people: the crude, sometimes brutal, behavior of police officers and other street-level law enforcers; and the treatment of children in a society where many parents have only one child. All the more striking, the scene took place in Shanghai, one of China’s richest, most sophisticated cities.

By Saturday, millions in China had watched and dissected the video on social media, especially on Sina.com Weibo, a microblog service like Twitter, and on WeChat, where users often share images and opinions with friends.