Vietnamese Facebook users had problems accessing the popular social network over the past weekend, apparently due to an ongoing protest that the government has been trying to suppress.

For the past two weekends in a row, hundreds of people have been protesting in Vietnam's two major cities of Hanoi and Ho Chi Minh City, as well as in smaller cities like Nha Trang, Vung Tau and Da Nang.

The protests were prompted by millions of dead fish washing up on its shores since April — which local media and protesters have blamed on Formosa Plastics, a Taiwanese firm.

Masses of dead fish on a Vietnamese beach in late April, 2016. Image: AFP/Getty Images

Formosa has denied responsibility. Vietnam's environment minister admitted the company had an illegal waste pipe at one of its steel plants in Vietnam, and was ordered to dig it up. He said the pipe hasn't conclusively been linked to the fish deaths, but protesters are seeing red over the firm.

Protesters on May 1, 2016. Image: HOANG DINH NAM/AFP/Getty Images

So over the weekend, when protesters were expected to rally for the third time, Facebook was inaccessible to locals, who had been using the platform to organise.

People also had problems accessing Facebook's Instagram service.

Israeli VPN service Hola posted a statement saying it saw a surge of about 200,000 users from Vietnam on its system over the weekend, using it to access Facebook.

Despite the block, rallies still continued, albeit in smaller numbers than in previous weeks, according to blogs. Protesters were also rounded up by the police.

#VietnamProtest -Bride and groom try to cross barbed wires set up around Saigon to deter demonstrators. May 15, 2016 pic.twitter.com/yHEyMOYEyZ — Saigonese (@VietNamTweeter) May 15, 2016

#VietnamProtest "Hit me in my face if you want, but give back the ocean and human rights to our people" -Hanoi woman pic.twitter.com/nhHyvwDcIG — Ian Bui (@ian7bui) May 13, 2016

Dead fish continue to float up in the rivers, according to the latest reports. On Tuesday morning, a major canal in Ho Chi Minh City was filled with dead fish. Authorities say 70 metric tons of fish have been retrieved in the 8.7 km Nhieu Loc–Thi Nghe canal alone so far since April.

The director of the agriculture department, Nguyen Phuoc Trung, told reporters the fish were killed by polluted water in the canal.

But this has happened in 2015 and 2014's May rainy seasons in the polluted canal. Nhieu Loc–Thi Nghe is connected to a sewage system that discharges wastewater into it, and is often stagnant.

We've reached out to Facebook for comment.

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