The brother of a prominent overseas Cambodian activist was arrested at the Thai border as he attempted to leave the country just weeks after Senate President Hun Sen publicly threatened the activist’s family.

Hay Vanna, a political activist who lives in Japan, told Radio Free Asia that his brother, Hay Vannith, was detained in Poipet in northwestern Cambodia on Aug. 16.

Hay Vannith was forced to make a written confession that he had participated in plans for nationwide protests in Cambodia last weekend, according to Hay Vanna, who added that his brother has never been a part of his political advocacy.

The planned Aug. 18 demonstrations against the Cambodia-Laos-Vietnam Triangle Development Area, or CLV, never took place as the government deployed police, military police, soldiers and special forces across the country.

A total of more than 30 people were arrested over the weekend as authorities set up checkpoints on highways and cities.

The 1999 CLV agreement between the three countries was aimed at encouraging economic development and trade between Cambodia’s four northeastern provinces and neighboring provinces in Laos and Vietnam.

But some activists recently began expressing concerns that the CLV could cause Cambodia to lose territory or control of its natural resources to Vietnam.

Earlier this month, overseas Cambodian activists – including Hay Vanna – organized protests against the CLV in South Korea, Japan, Canada and Australia.

Last week, activists formed a chat group in the Telegram app to organize protests in Cambodia. But that prompted Senate President Hun Sen last week to issue a warning of widespread arrests of activists.

‘Stop or else’

Last month, Hun Sen called out Hay Vanna by name in a speech broadcast on state-run television.

“This person by the name of Hay Vanna who lives in Japan, commented on the so-called ceasing of the four Cambodian provinces to others,” Hun Sen said on July 23.

“But you shouldn’t be confused – you have family members here in Cambodia,” he said. “And they who are living here, must not be arrogant. After hearing his message ... you must stop, or else.”

Family members haven’t been able to contact Hay Vannith, according to Human Rights Watch, which said the 28-year-old civil servant was “forcibly disappeared.”

Hay Vannith studied in the United States as a Fulbright scholar and now works for the Ministry of Health, according to the Manushya Foundation, a Bangkok-based human rights group.

“He was trying to flee to Thailand due to the threats he was facing,” the foundation said.

Hay Vanna told RFA that the government has taken his brother hostage.

“I am not afraid or worried even if they arrest my brother,” he said. “I won’t stop my activities.”