The country's authoritarian leaders keep a very tight grip on public gatherings for fear they could attract anti-government protesters.
This time, they appeared to give in to public anger that also provided them with the opportunity to register their own indignation at Beijing.
Other protests, including one drawing more than 1,000 people in Ho Chi Minh City, took place in other locations around the country. For the first time, they were reported on enthusiastically by the state media.
"We are infuriated by the Chinese actions," said Nguyen Xuan Hien, a lawyer who printed his own placard reading "Get Real. Imperialism is so 19th century."
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"We have come to so that the Chinese people can understand our anger," he said.
Vietnam's government immediately protested the deployment of the oil rig on May 1, and dispatched a flotilla that was unable to break through a circle of more than 50 Chinese vessels protecting the facility. The Vietnamese coast guard released video of Chinese vessels ramming and firing water cannons at Vietnamese ships.
China claims sovereignty over the area and most of the South China Sea a position that has brought Beijing in confrontation with other claimants, including the Philippines and Malaysia.
The protest today was the largest since 2011, when a Chinese vessel cut seismic survey cables leading to a Vietnamese oil exploration ship. Vietnam sanctioned protests for a few weeks, but then broke them up after they became a forum of anti-government sentiment.