Human Rights Watch (HRW) on Tuesday demanded the Cambodian government release three environmental activists arrested last week in for documenting the dumping of raw sewage into a river.

The nonprofit also demanded the withdrawal of "baseless conspiracy and 'insulting the king' charges" against the three, affiliated with the environmental group Mother Nature Cambodia, which was founded by Spanish national, Alejandro González-Davidson.

Lèse-majesté (insulting the king) charges are punishable with between five and 10 years in prison.

The activists were arrested after they filmed sewage in the river near Royal Palace, the Cambodian Center for Human RIghts said.

"The Cambodian government has stepped up its campaign to silence activists peacefully advocating to protect the environment,” Phil Robertson, deputy Asia director, said in a statement.