Vietnamese police have detained dozens of people who were taking part in an anti-China rally Sunday in the capital of Hanoi.

Just minutes after the protest began, plainclothes agents moved in and forced at least 40 demonstrators onto waiting buses and drove them away.

Thursday the Hanoi People's Committee had ordered a halt to the rallies, warning the government would apply “necessary measures” against those who failed to comply.

The decree described the protests as spontaneous and acknowledged they are motivated by patriotism. But it said they were adversely affecting social order and the city's image, while negatively impacting diplomatic activities of the ruling party and state.

Demonstrators have turned out in Hanoi every Sunday for about 11 weeks to protest Chinese actions in the South China Sea. The government says Chinese vessels have deliberately interfered with oil exploration activities in disputed waters off Vietnam's shores.

The announcement charged that in recent days, unidentified “opposing forces” within and outside the country have been inciting and guiding the demonstrations. It demanded that the participants stop all activities and gatherings in the city.

In most cases over the past few weeks, police simply watched the protests, suggesting authorities approved of the action. But the demonstrations were broken up on a few occasions while the government was conducting delicate negotiations with China on the issue.