.Alejandro Gonzalez-Davidson, co-founder of the outspoken advocacy group Mother Nature, has led a campaign against plans for a dam in a protected forest area in the southwestern province of Koh Kong.
He will be expelled from the kingdom after refusing to leave when his visa renewal application was refused, interior ministry spokesman Khieu Sopheak told AFP.
"We have asked him to leave the country (voluntarily) but he has refused to do so. Therefore we must expel him," Sopheak said, adding the activist's previous visa expired Friday.
Gonzalez-Davidson, a fluent Khmer speaker who has lived in Phnom Penh for years, has been lobbying to halt the construction of Stung Cheay Areng hydroelectric dam.
His organisation claims it would flood a valley home to around 1,500 ethnic minority people, destroying a unique ecosystem.
Sopheak said the activist's visa was not renewed because of complaints filed against him by the energy ministry and the Koh Kong governor.
But Am Sam Ath of local rights group Licadho said the move to expel Gonzalez-Davidson was directly linked to his vocal opposition to the proposed dam.
The Spaniard was arrested at a restaurant in the Cambodian capital and is being detained at the immigration department, he added.
"The arrest will harm the government's reputation," Ath said.
Gonzalez-Davidson was detained hours after strongman premier Hun Sen warned the defiant activist to leave Cambodia voluntarily or face being blacklisted from the country.
Hun Sen, who marked 30 years in power in January, has said the economic benefits of a series of controversial dams outweigh environmental concerns in a country where a quarter of the population still lacks electricity.
Nine dams, including several funded by China, are set to open by 2019.
Once they are operational the government has said they will together generate more than 2,000 megawatts, serving all of Cambodia's provinces.