The government has continued its crackdown on activists demanding transparency over the mass death of fish in central provinces.
Police detained more than thirty people as they tried to stage protests in Hanoi and Ho Chi Minh City.
The government’s failure to account for the disaster, two months after tonnes of dead fish began washing up on beaches, has fuelled frustration and suspicions of a possible cover-up.
Officials said last week that they had completed an investigation to identify the cause of the fish deaths but needed to consult with experts at home and abroad before releasing the findings.
An initial government report in April failed to identify the cause of the contamination and said there was no evidence to implicate the Taiwanese company Formosa, which has been the focus of public anger over the environmental disaster.
Activists suspect that discharges from the company’s steel plant under construction in Vung Ang on the north-central coast were responsible for the mass poisoning of fish.
Security forces took to the streets of Vietnam’s two largest cities in force as they sought to prevent a repeat of previous demonstrations called to demand accountability.
Some activists were detained before they were able to get close to the site of the proposed rally in front of the cathedral in Hanoi.
The police response appears to be getting tougher each week following marches by up to a thousand demonstrators in the first demonstrations.
Human rights campaigners say the government’s response highlights its refusal to tolerate criticism or accept even minimal levels of public accountability.
The heavy-handed crackdown on protests, and the continuing delays in the investigation, have fuelled speculation about a possible cover-up and the extent of links between Taiwanese investors and Communist party officials.