Scores of security officers are monitoring the demonstrators as they shout anti-Chinese slogans and hold up banners.
Vietnam's authoritarian leaders are highly nervous about public protest even as they have condemned China's decision to send the rig. Dissident movements opposed to their rule have also taken part in previous anti-China protests.
Hanoi dispatched a flotilla of ships to the oil rig soon after it was deployed on May 1, and say some were rammed by Chinese boats. The latest sea confrontation in the disputed waters is raising fears that tensions could escalate.