Her arrest comes as the communist nation is accused of waging a crackdown against dissidents, with more than a dozen people arrested this year and others handed heavy jail terms.

Tran Thi Xuan, 41, is accused of “activities aimed at overthrowing the people’s administration” under Article 79 of the criminal code, according to the official mouthpiece of Ha Tinh province, where she was arrested Tuesday.

The article is one of several vaguely-worded clauses of the penal code that rights groups say are used by the communist country’s authoritarian rulers to persecute critics.

Those found guilty of violating Article 79 could face death.

Xuan’s brother Tran Quyet Tien said she was involved in a protest last year against Taiwanese steel giant Formosa, which was blamed for a toxic dump that killed tonnes of fish along Vietnam’s central coast.

He said this was the third time she had been detained, but added he did not know why police took her.

“I’m very sad. But I’m also frustrated… my sister is innocent and it’s ridiculous that the police arrested her,” Tien told AFP.

The disaster in April 2016 decimated livelihoods in fishing villages along the central coast, including in Ha Tinh province where Xuan is from.

Formosa, which was building an $11 billion steel plant at the time of the fish deaths, was fined $500 million over the disaster.

But protests have continued in coastal communities since the fish deaths, with authorities cracking down on the demonstrations and arresting leaders.

Vietnam routinely jails activists, bloggers and lawyers, and has come under fire for a spate of arrests this year.

Amnesty International says at least 15 activists and critics have been jailed since January.

Four dissidents were arrested in July on charges of trying to overthrow the state.

They were all connected to an activist and a lawyer who were already detained on the same charge.

Others have received harsh jail sentences in recent months.

In June, prominent blogger Nguyen Ngoc Nhu Quynh, known as “Mother Mushroom”, was jailed for 10 years for Facebook posts about politics and the environment.

A few weeks later, anti-China activist Tran Thi Nga was imprisoned for nine years for anti-state activities after a one-day trial.

Some analysts say Vietnam is cracking down on activists ahead of an Asia-Pacific Economic Cooperation summit in November, which will be attended by global leaders, including US President Donald Trump.

His administration has been accused of putting human rights concerns on the backburner while seeking overseas trade deals.

Agence France Presse - October 18, 2017