The police have broken up demonstrations in Hanoi and Ho Chi Minh City which were called to protest against the government’s response to the environmental disaster off the central coast.
Witnesses said as many as 200 protesters appeared to have been detained in Ho Chi Minh City.
Marchers carrying banners and shouting slogans were surrounded by officers and bundled into buses before being driven to the Hoa Lu sports stadium near the city centre.
The area around the stadium was sealed off by police.
Video footage from the scene showed officers beating some of the protesters, who were marching to demand government transparency over a disaster that has seen tonnes of dead fish wash up on beaches in central provinces.
A number of the injured were taken to hospital.
About 100 people were also reported to have been arrested in Hanoi. The police rounded up the protesters before they could begin their march, detaining many leading activists in the growing campaign to demand more government accountability.
The government has yet to announce the cause of the disaster more than a month after piles of dead fish began appearing along 200 kilometres of shoreline.
Environmental activists suspect the sea was contaminated by waste from a steel plant being built by a subsidiary of the Taiwanese Formosa group.
They are demanding a more energetic response from the government, full accountability, and support for fishermen who have lost their livelihoods.
The government’s violent response to the latest demonstrations shows its growing alarm at the public anger directed at officials.
Activists have used social media to give copious coverage of the environmental disaster as it unfolded.
The state controlled media have also been unusually active in covering the story, which is seen as a major challenge for the newly installed administration of the prime minister, Nguyen Xuan Phuc.
Official media, however, have barely mentioned the street protests and the calls for action from civil society groups and independent bloggers.