HANOI (Reuters) - Hundreds of people demonstrated in Vietnam on Sunday (May 1) against a Taiwanese firm they accuse of causing mass fish deaths along the country's central coast, with some also blaming the government for a sluggish response to a major environmental disaster.

Though an official investigation has found no links between the fish deaths and a US$10.6 billion coastal steel plant run by a unit of Taiwan's Formosa Plastics, public anger against the company has not abated.

Hundreds gathered in Hanoi holding banners that said: "Formosa destroying the environment is a crime" and "Who poisoned the central region's waters?"

Others said: "Formosa out of Vietnam!" and took aim at the government for being aloof in what it now describes as one of its worst environmental disasters.

Demonstrations are rare in Vietnam and uniformed and plainclothes police are usually quick to suppress them. On Sunday they cleared traffic to allow demonstrators to do a lap of a big lake in the heart of Hanoi.

Huge numbers of dead fish have appeared at farms and on beaches since April 6, impacting 200km of coastline in four provinces, with no known cause.

The environment minister has demanded Hung Nghiep Formosa Ha Tinh dig up its waste pipe at the steel project to enable government to monitor its discharge.

The government's initial probe said the cause could be "red tide", when algae blooms and produces toxins, or a release of toxic chemicals by humans.

What has stoked public anger was a comment by a Formosa official who said Vietnam had to choose between catching fish and shrimp and building a modern steel industry.

"Here is Vietnam's territory and there shall never be any case in which a Formosa steel plant has the right to tell the Vietnamese people to choose," protester Cao Vinh Thinh said.

Several hundred protesters marched in Ho Chi Minh City, the economic hub, according to multiple accounts on Facebook, which is used by 30 million Vietnamese.

State-controlled media has not reported any of the demonstrations.

Social media and witnesses said protests also took place in central Quang Binh province on Friday, with fishermen throwing fish on the highway after failing to sell their catch. The accounts could not be verified by Reuters.

The government on Saturday ordered the trade and agriculture ministries to help buy seafood caught during deep-sea fishing.