Dozens of activists were detained in Vietnam's two biggest cities Sunday as they tried to hold protests calling for greater government transparency over a recent spate of mass fish deaths.

Tonnes of dead fish and other marine life began washing up on central Vietnamese shores two months ago and continued to appear for two to three weeks, sparking widespread anger.

Frustration has been further fuelled by a perceived lack of clarity from the communist leadership about what caused the deaths.

Major streets in central Hanoi and Ho Chi Minh City were temporarily deserted on Sunday morning as security forces blanketed the area.

Activists used social media accounts to document many arrests, saying people had been detained in both cities either before or while they joined protests.

AFP saw one incident in Hanoi where plainclothes officers tackled an activist and dragged him off.

Another 30 people were detained after launching a brief protest in front of Hanoi's cathedral.

Activists posted pictures showing the protesters holding banners with slogans like "Fish dead, people dead" and "No Formosa".

Taiwanese conglomerate Formosa, which runs a steel mill in the area where the fish have died, is widely suspected of being the cause.

State media initially pointed the finger of blame at the company but have since rowed back, as anger grew in Vietnam against ethnic Chinese interests in the country.

Authorities have since responded with a crackdown against protesters, placing many activists under house arrest and at times interrupting the Internet.

Facebook has been intermittently unavailable since Saturday night.

Human rights groups hit out at the latest crackdown.

"The Vietnam government should be leading the investigation into what caused these fish kills, not suppressing people's efforts to demand answers and accountability," said Phil Robertson from Human Rights Watch.

On Thursday government official Mai Tien Dung was quoted in state media as saying that investigators have now concluded what was killing the fish.

But they would only release the results after "independent consultation from domestic and foreign experts", he said.

Fishing and tourism in central Vietnam have been hit hard by the marine deaths.

Vietnam's communist rulers tolerate little dissent but anger over corruption and environmental degradation often spark significant protests.