Cambodia deported a Spanish environmental activist Monday after he was arrested in Phnom Penh, officials said, a move described by a rights group as the government's latest attempt to stifle dissent.

Alejandro Gonzalez-Davidson, co-founder of the advocacy group Mother Nature, has been an outspoken and long-time campaigner against plans for a controversial dam in a protected forest area in the southwestern province of Koh Kong.

Interior ministry spokesman Khieu Sopheak told AFP earlier that authorities were compelled to "expel" him as he refused to leave the country voluntarily after his visa renewal application was refused.

Gonzalez-Davidson, a fluent Khmer speaker who has lived in Phnom Penh for years, was put on a plane bound for Thailand late Monday, an immigration official at the capital's airport, who asked not to be named, told AFP.

The Spaniard, who has been lobbying to halt the construction of Stung Cheay Areng hydroelectric dam, urged Cambodian nature lovers to "stay strong" in a text message to his group before being deported.

"The battle is yours to be won. For Nature, our Life," he said, according to a statement released by Mother Nature.

The organisation claims the proposed dam in Koh Kong would flood a valley home to around 1,500 ethnic minority people and destroy a unique ecosystem.

Sopheak said Monday that 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 deportation was directly linked to Gonzalez-Davidson's vocal opposition to the proposed dam.

"The arrest will harm the government's reputation," Ath said.

Gonzalez-Davidson was arrested at a Phnom Penh restaurant on Monday and detained just 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.