The protest in Hanoi, which follows a similar demonstration last weekend, was swiftly dispersed by authorities on Sunday morning, an AFP reporter witnessed, in a communist country where all shows of dissent are tightly controlled.
|Scores held as Vietnam breaks up fish deaths protest|
"Never has the Vietnamese sea been this badly polluted," army veteran Nguyen Manh Trung, 68, told AFP.
But "the police are now more and more professional in breaking up protests," he added of the scores of people taken away in unmarked cars.
Vietnam's prime minister has vowed to get tough on those responsible for the leak, but an official inquiry has yet to apportion blame.
However state-run media has pointed the finger at a 1.5 kilometre (one mile) waste water pipeline from Formosa's multi-billion dollar steel plant into the ocean.
The company has a bad record of environmental scandals spanning the globe. But it has not formally been linked to the mass fish poisoning.
As the scandal unfolded in April a Formosa communications official was sacked after he said Vietnam needs "to choose whether to catch fish and shrimp or to build a state-of-the-art steel mill".
"You cannot have both," the official said.
The company later apologised for the comments and has launched its own inquiry but public anger is snowballing.
Vietnam's central provinces are heavily dependent on seafood, including farmed shrimp, catfish and wild-caught tuna.
Last year the country earned $6.6 billion from seafood exports.