Last Updated, Thursday 3:06 p.m. On Tuesday in Beijing, amid protests over the sovereignty of several tiny islands also claimed by Japan, a small group of protesters briefly surrounded the American ambassador’s car outside the United States Embassy, pelting it with objects. As our colleagues Thom Shanker and Ian Johnson reported, Ambassador Gary Locke said later, “I never felt in any danger.”

Video of the incident, recorded and uploaded to YouTube by the dissident artist Ai Weiwei, showed that the protesters arrived at the embassy’s gate, with remarkable timing, at precisely the same moment as the ambassador’s car. A line of police officers appeared, running in formation, seconds before the protesters blocked the car.

The video clip also showed the protesters, who had apparently peeled off from a larger anti-Japanese rally, chanting and throwing objects at the car, before police officers cleared a path for the vehicle to drive off.

Writing on Twitter, Mr. Ai reported that dozens of protesters had arrived at the embassy at about 4 p.m. chanting, “Pay us back our money!” and “Down with U.S. imperialism,” perhaps in reference to American support for Japan.

The artist also posted a series of photographs on Twitter, via Instagram, which appear to have been shot just after the video, from above and at street level, showing more and more security gathered around the embassy gate.

Mr. Ai, via Twitter, confirmed to The Lede that he shot the video and photographs of the incident and shared them on the social networks. Earlier this week, another clip posted on YouTube by a video blogger in Beijing showed the artist shooting images of a protest on Sunday outside the Japanese embassy on his iPhone.

In April, our colleague Edward Wong reported, Mr. Ai said that he was ordered to shut down a Web site, weiweicam.com, which streamed live video of the artist from webcams in his home and studio in Beijing. That project was apparently a critique of the government’s decision to put him under constant surveillance after releasing him from 81 days in secret detention.

As the American filmmaker Alison Klayman explained in a PBS Frontline video report on Mr. Ai broadcast last year, he has experimented with citizen journalism in the past. After an earthquake devastated Sichuan Province in 2008, the artist gave video cameras to readers of his blog in the region, to begin what he called a citizens’ investigation into the earthquake deaths.

Watch Who’s Afraid of Ai Weiwei? on PBS. See more from FRONTLINE.