Protesters have staged rallies across the country to demand the expulsion of the Taiwanese company, Formosa, for causing last year’s devastating chemical spill off the central coast.
The largest demonstrations was reported in Nghe An province, where residents said several thousand people from Catholic parishes gathered after Sunday mass.
Squads of police took up positions close to the rally but did not intervene.
A crowd also gathered outside the Formosa steel plant itself, further south in Ha Tinh province, to express their anger at the handling of the toxic leak and its aftermath.
Fishermen along the coast have suffered huge losses following the mass poisoning of fish last April and continuing doubts about the safety of marine products in the contaminated areas.
A smaller gathering of protesters in Ho Chi Minh City was quickly broken up by police. About two hundred people in small groups near Notre Dame Cathedral were dispersed, and several people were arrested, after they started shouting anti-Formosa slogans.
Police in Hanoi managed to prevent activists assembling after a call for a rally was posted on social media.
The call for nationwide protests came as residents’ groups and environmental activists try to step up their campaign against the government’s handling of the disaster, which severely damaged the fishing industry along some 200 km of the north-central coastline.
Many residents complain they have not received any compensation, despite an admission of guilt by Formosa last June and its offer of $500 million for those who suffered losses.
The authorities have taken an increasingly hard line with protesters, resorting to violence to break up some demonstrations. Last month, police used baton charges and mass arrests to halt a march by residents from Nghe An who wanted to submit lawsuits against Formosa.