Outspoken environmental activist Alex Gonzalez-Davidson and San Mala, a colleague at his NGO Mother Nature, have been arrested by Interior Ministry officers and taken in an unmarked car to the Department of Immigration, a rights group and witnesses have said.
Cheang Sophos, a senior investigator for local rights group Licadho, said that Gonzalez-Davidson was arrested at about 1:15pm at the Fish & Co restaurant in the riverside area of Phnom Penh.
The news of both arrests was confirmed by a spokesman for the Community Legal Education Centre and a witness at the scene.
Prime Minister Hun Sen earlier in the day warned the defiant activist to leave Cambodia voluntarily or be blacklisted from the country.
Gonzalez-Davidson - who is facing deportation after the government refused to renew his visa - has vowed to remain in the country despite the fact that his visa expired on Friday.
Speaking at a graduation ceremony in Phnom Penh on Monday, the premier also warned NGOs not to rally behind the cause of the embattled co-founder of Mother Nature, lest they face problems of their own.
"Regarding Alex [Gonzalez-Davidson], let the Ministry of Interior take measures. It's not just foreigners, it's also Khmers that will be sentenced and other NGOs shouldn't express much," Hun Sen said.
"We'll let him stay until his visa is invalid. So you should leave first then ask for a new visa, it doesn't matter. [You] don't need to make this situation get worse. If we deport you, it means [you're] on the blacklist, that's it".
Gonzalez-Davidson is legally entitled to stay for 37 days after his visa expiration provided he pays related fines, but he has vowed to remain in Cambodia indefinitely.
A staunch advocate on environmental causes, particularly that of the threatened Areng Valley, Gonzalez-Davidson has previously told the Post he is certain that if he leaves the country to obtain a new visa, the government will not let him back in.
His situation has attracted huge support on social media, where the fluent Khmer-speaking activist has become something of celebrity.
On Monday morning, Gonzalez-Davidson was unavailable for comment, but San Mala, a co-founder of Mother Nature, said support for his colleague in the Cambodian community was absolute.
"Hun Sen is going against what the Khmer people want, as we know 100 per cent of Khmers want Alex to stay. [Hun Sen's actions are] opposite to Khmer people," he said, pointing out that Alex has grown to be considered "Khmer" by his supporters.
"Our team won't let Alex leave this country, as he is Khmer, lives here, can speak Khmer like a native and, most importantly, is the one who helps to protect the Areng Valley from illegal logging and the hydropower dam".