BANGKOK — ASEAN, or the Association of Southeast Asian Nations, is not an acronym that tends to excite. Founded on notions of consensus-building and noninterference, the 10-nation group issues pronouncements drowning in careful and bland language.

This week, though, regional governments have banded together to use ASEAN’s principles to justify a startling act: blocking Cambodian opposition politicians who are trying to return home to confront Prime Minister Hun Sen of Cambodia, Asia’s longest-serving authoritarian leader.

On Wednesday night, Mu Sochua, the deputy leader of a political party that came closest to unseating Mr. Hun Sen in earlier elections, was detained at the airport after arriving in Malaysia, according to Phil Robertson, the deputy Asia director for Human Rights Watch.

A dual citizen, Ms. Mu Sochua tried to enter Malaysia on her American passport, said Mr. Robertson, who was in touch with Ms. Mu Sochua during her travels. She was held overnight in detention at the Kuala Lumpur International Airport, he said.